Zenyatta:  Zenyatta's slow movement and poor mobility makes sniping him fairly easy for Widowmaker.  Zenyatta should also be a prime target since he is a very squishy hero that also heals his allies.

Widowmaker shouldn't be a prime target for Zenyatta's Discord Orb, since she is not in the front line pushing forward and instead stays back relatively hidden from her enemies.  Widowmaker can use her Grappling Hook to quickly get behind a wall to remove the Discord Orb, but this will disable her for a short period of time as well, which can be bad for snipers.

Zenyatta generally does not come into contact with Widowmaker's Venom Mines since they are usually a distance from each other.

When Zenyatta uses his ultimate Transcendence, try to locate and snipe enemies that are away from him at that given time.  If there are no other options, just shoot anyways to get ultimate charge.

Ana:  Both Widowmaker and Ana are snipers, but Widowmaker simply does a lot more damage, which can out DPS Ana's Biotic Rifle by 40%, this does not include the fact that Widowmaker can do headshots that do x2.5 the damage also, while Ana does no headshots.  Widowmaker's Infra-Sight only makes it much easier for her to snipe Ana to death when Ana pops in view.

Ana's Sleep Darts are usually ineffective against Widowmaker, the two heroes are usually too far away from each other and any good Widowmaker player will strafe side to side to make her a harder target for enemies to hit her, especially from Sleep Darts.  Do not stay stationary or Ana will have a much easier chance throwing a Sleep Dart on you.  Even with the strafing, there is a slim chance of Ana hitting Widowmaker with a Sleep Dart, if so, disabling Widowmaker's sniping from combat for a short while is very bad for your team.  Also, if Widowmaker gets hit by a Sleep Dart while using Infra-Sight, the Infra-Sight ability will continue while Widowmaker is sleeping.  Ana must make tough decisions whether or not to throw a Sleep Dart at Widowmaker during combat.

Ana's Biotic Grenade tends to be easier to hit Widowmaker than Sleep Darts and it could do some extra damage to you while also disabling any healing on you at the time as well.

Since Ana keeps her distance from enemies by staying in the backlines, she is generally safe from Widowmaker's Venom Mines.

Ana can use her ultimate Nano Boost on one of her allies (especially a flanker such as Tracer, Reaper, Genji or D.va), which can let them have the extra power and protection they need to quickly get to Widowmaker to take her out.  Widowmaker's only defense against that is hope to grapple away from it.

Pharah:  Pharah is easy to snipe since she is mostly flying around in the air slowly with her Jump Jets.  You just need to watch out for her rockets and you will most likely need to reposition yourself continuously with your Grappling Hook.  Dodging Pharah's ultimate Barrage should be easy for Widowmaker since she can easily grapple away from it, but you will most likely not be targeted from it anyway.

There is a slight chance that Pharah could flank Widowmaker due to her superior mobility, so keep your eyes and ears glued to the game to make sure you don't get flanked by a Pharah.  Your teammates should also warn you if a Pharah took a sneaky route to get around for flanking purposes.

Venom Mines are virtually useless against Pharah since she is usually flying in the air most of the time.

Hanzo:  Sniper Vs Sniper?  This is an interesting matchup and I believe that Widowmaker has the advantage here simply because her sniper abilities are simply more superior in long range snipe battles.  However, the closer Hanzo gets to Widowmaker the easier it will be for him to take her out.  Since Widowmaker is usually way in the back from her team and away from Hanzo, she doesn't need to worry too much about a Hanzo shooting her down (even though it can happen).  There are numerous places in Overwatch that Widowmaker can use her Grappling Hook to get up to that Hanzo cannot climb to, thus giving her more of a safe advantage over Hanzo in those situations.

On the bright side for Hanzo, he doesn't usually need to worry about Widowmaker's Venom Mines, since her mines are mostly used for dealing with offensive heroes that try to get close to her team.  Hanzo's ultimate Dragonstrike happens to be somewhat useful on Widowmakers, since it can force you to leave your current sniping spot to find another one, temporarily disabling you from combat for a short period of time, which can be huge.

McCree:  Even though McCree's revolver does instant bullet shots (his bullets have no travel time), it does have fall-off distance damage.  This means he will most likely do much less damage to Widowmakers that are sniping from a distance.

McCree becomes more deadly when paired up with heroes such as Reinhardt and his shield, as they can push toward Widowmaker while blocking her shots.  If McCree actually manages to get right up close to Widowmaker it could be an easy kill for McCree with his Flashbang and fan fire attack.  But Widowmaker can always grapple away to save herself as McCree has very poor mobility and won't be able to keep up.

If a McCree uses his ultimate Deadeye, it's usually Widowmaker's job to quickly headshot him as he will just stand there and be an easy snipe target for her.  NOTE:  Enemy McCrees will yell "It's High Noon" when he uses Deadeye, while ally McCrees will say "Step Right Up".

Mercy:  Mercy is relatively easy prey for Widowmaker.  Even though Mercy is good at taking cover most of the time, when the opportunity strikes, she is easy to snipe since she moves around quite frequently to her allies and sometimes even floats around in the air with her Guardian Angel ability.  Sniping Mercy should be one of your first targets to snipe as Widowmaker, as taking her out dramatically helps out with delaying her Resurrect ultimate and halts her insane healing ability.

Widowmaker's Infra-Sight ultimate works really well against Mercy.  Since Mercy hides around walls most of the time, Widowmaker and her team can predict when she comes within view much easier with Infra-Sight.

Venom Mines are virtually useless against Mercy since she is usually in the back lines with her team most of the time.

Junkrat:  Widowmaker is virtually safe from Junkrat's traps, mines and Total Mayhem death bombs since she is always way in the back sniping from a distance.  Although Junkrat can take cover while lobbing grenades, he is usually out of reach to aim his Frag Launcher at Widowmaker, and if Junkrat peeks in view, it's GG for Widowmaker.  Widowmaker's Infra-Sight only makes it easier for her to see when a Junkrat pops into view for her to shoot down.

Since Widowmaker is usually far away from Junkrat and can get up high with her Grappling Hook, she is usually safe from Junkrat's Rip-Tire.  Although, Junkrat usually doesn't need to worry about coming in contact with Widowmaker's Venom Mines (although it can happen) since they are mostly used to keep offensive enemies from pushing toward Widowmaker's team.

Bastion:  Due to a Bastion Sentry's stationary nature, you can easily aim sniper shots at him from very long ranges and it will be harder for a Bastion Sentry to do a lot of damage to you due to his decreased accuracy at long ranges.  Sniper shots at Bastion Sentries are very effective and it will force him to spend more time healing and finding different spots to setup a Sentry defense.

It is possible to flank Bastion from behind as Widowmaker, if you can be sneaky and grapple behind him without him noticing you, it could be an easy kill for you then.  However, this rarely happens and you are usually better off just keeping your distance from him and sniping from afar anyways.

Infra-Sight can be slightly effective against Bastion Sentries as it will reveal his camping spot to your whole team, making Bastion more vulnerable overall.

It is possible to throw a Venom Mine right next to a Bastion Sentry, but you will have to get somewhat close to him, which can prove to be a difficult challenge.

Widowmaker is generally in the clear of Bastions Tank ultimate since she is usually far in the back taking cover while her team will soak up the tank damage.

Mei:  Widowmaker rarely makes close contact with her enemies since she is normally in the back sniping, because of this she never needs to worry about getting frozen by Mei's freeze gun.  If Widowmaker does happen to get close to Mei, she can easily use her Grappling Hook to escape to a safer spot.

Mei can throw up an Ice Wall right in front of Widowmaker to block her shots temporarily.  This could force Widowmaker to relocate temporarily disabling her from combat for a brief period of time.

Widowmaker is less effected by Blizzard since she is usually by herself sniping, away from the heart of the battle.

Zarya:  Zarya is virtually harmless against Widowmaker.  Her Particle Cannon usually doesn't reach far enough to damage Widowmaker, even with her explosive charges, which Widowmaker can usually strafe to avoid or grapple to a different sniping spot.

Widowmaker is also usually safe from Zarya's ultimate Graviton Surge since she is usually away from the heart of the battle and off sniping by herself.  If you do get stuck in a Graviton Surge, you can simply escape out of it by grappling.

Remember, when Zarya uses her Particle Barrier, either on herself or her allies, don't shoot at it or it will absorb your shots and also make Zarya's weapon more powerful.

Tracer:  Fast moving targets are very annoying for Widowmaker, thus Tracers are very hard to hit for her, in all ranges.  Widowmaker is also a prime target for Tracer and Widowmaker will need to watch her back from getting flanked by her.  Use Venom Mines in clever spots leading up to your sniping spot to hold back enemies such as Tracer.  Although, Tracer can Blink past a Venom Mine unharmed.  Keep grappling around to different sniping spots to confuse Tracer on your whereabouts.

If you hear Tracer use her ultimate Pulse Bomb, immediately grapple away from your current spot, you may be able to escape it as long as she didn't stick the bomb on you.  NOTE:  Enemy Tracer's will yell "Times Out" when she throws out a Pulse Bomb, while an ally well say "Right on Target".

Reaper:  Reaper's sneaky mobility allows him to get around the map and kill Widowmaker unnoticed.  Reaper can Shadow Step (teleport) behind Widowmaker and then flank her.  Reaper can use his Wraith Form to save himself from Venom Mines or other deadly situations.

Dealing with Reapers can be difficult.  My best tip is to keep your eyes and ears peeled on the game for incoming Reapers.  Memorize the sounds that Shadow Step uses and you will do much better against him.  Reaper's Shadow Step has a slight delay once he teleports in, this gives Widowmaker a widow of opportunity to headshot him once he starts moving again.  Memorize the sounds of Reaper's footsteps, this can alert you to exit sniper mode and grapple away from him.  Put down Venom Mines around you and if Reaper runs over it he may take some damage before he turns on Wraith Form, this will give you enough time to grapple to a new destination to save yourself.

Genji:  Genji is a great counter to Widowmaker.  His parkour abilities (climb walls and double jump in the air) enables him to sneak around the map and flank Widowmaker unnoticed.  Up close, Genji's deadly Swift Strike/Shuriken combo is very deadly to Widowmaker and if she attempts to shoot him, Genji can use Deflect to ricochet the bullets right back to damage her.

From long distance, be careful when firing at Genji, because he can Deflect your sniper shots right back at you, which can be an easy kill for Genji.  When sniping at Genji while he is approaching you, keep strafing to decrease the chance deflected bullets will hit you.  Pay close attention and do not fire at Genji when he has Deflect initiated.  If you come in close contact with Genji, quickly throw a Venom Mine his direction and grapple away to a safe spot.  By doing so, you will be protected from his deadly combos or ultimate Dragonblade.

Genji CANNOT Swift Strike past a Venom Mine to avoid it, as it will still trigger on him.

Infra-Sight is very useful for tracking down if a Genji is getting closer to flank you.  If you see him approaching, quickly start grappling and working your way to somewhere safer.

Winston:  Winston's Barrier shields are powerful against Widowmaker for blocking all her shots temporarily protecting Winston and his team.  Winston can use his Jump Pack to leap right to Widowmaker and deal damage with his close range Tesla Cannon or turn on his ultimate Primal Rage to knock her around.  Winston's high HP means it will take a lot of good shots to take him down.

To counter Winston's close up leap, simply throw a Venom Mine at him and then grapple away before he does any further damage.

Lucio:  Since Lucio is a strong support hero with incredible healing abilities, he should be a prime target for Widowmaker.  Lucio's fast movement ability makes it somewhat harder to snipe him as he is skating around, especially when he has his speed aura on.  However, when Lucio uses his heal aura he will be a much easier target for Widowmaker to snipe.

Since Widowmaker is usually much further than Lucio, she doesn't need to worry too much about his Sonic projectiles as she can simply strafe to avoid it, or grapple to a different sniping spot.

If you are on a Payload map and Lucio is on Attack, he will most likely stay behind the payload.  A good strategy is when you have a clear open chance, go stick a Venom Mine on the Payload.  Lucio will have a good chance of hitting it, which can make taking him out much easier then.  Infra-Sight can help you to see how well guarded the Payload is.  And if you can, sticking the Venom Mine underneath the payload is a very good tactic (can be difficult to do though).

Roadhog:  Roadhog is not much of a threat to Widowmaker, but his high HP and powerful self healing ability makes taking him down quite difficult.  Luckily Widowmaker is usually out of range of Roadhog's Chain Hook.  Although, if Roadhog actually manages to Chain Hook Widowmaker toward him, she will need to quickly grapple away from him to avoid further damage.  And if you can, throw a Venom Mine before grappling away to do some extra damage.

Roadhog's Scrap Gun doesn't have the best range, even with his ultimate Whole Hog, so Widowmaker usually doesn't take too much damage from him overall.

Symmetra:  Most of the time Symmetra is putting up Sentry Turrets and Teleporters and is usually out of view of Widowmaker.  The only times when Symmetra may pop into view is when she lobs her slow moving Photon Orbs.  This is when Widowmaker has the opportunity to take a sniping shot at her.  This especially becomes easier with Infra-Sight, since it takes Symmetra a good amount of time to charge up the shots and then pop into view to fire them, giving Widowmaker a chance to line up a good shot at her.

Other than that, not much else happens between the two.  Widowmaker rarely encounters Symmetra's Sentry Turrets or Teleporters since she is too busy sniping and grappling away from all that.  And Symmetra will rarely run into a Venom Mine, unless Symmetra is on attack and Widowmaker puts a Venom Mine in a room where Symmetra may put up her turrets later.

Like I said above on the Infra-Sight section, Widowmaker can counter Symmetra's Teleporter by detecting her with Infra-Sight right when she places her Teleporter down, read more about that above at the Infra-Sight section.

D.va:  D.va can do a strong combo against Widowmaker.  She can use her Boosters to boost toward Widowmaker while using Defense Matrix (blocks incoming shots) at the same time.  This allows D.va to get right up close to Widowmaker unharmed.  Up close, D.va's Fusion Cannons are very powerful.  This can be deadly.  To counter this, you will simply need to relocate via grappling to a new spot.  Before you grapple away, quickly throw a Venom Mine on her to deal some extra damage.

D.va is a relatively large target that moves slow when she is firing, so sniping her at long ranges is very easy.  D.va's ultimate Self-Destruct is not too effective against Widowmaker since she can just grapple away from it and easily take cover.  When D.va gets ejected from her mech she can be a tough target to snipe due to her small hitbox, but definitely try to take her out before she calls down another mech.

Reinhardt:  Reinhardt's massive shield can pose a threat to Widowmaker, blocking her sniping shots while Reinhardt and his team can push forward.  This can be annoying and can force Widowmaker to relocate to a different spot to try and get open potential sniping shots in.

When facing a Reinhardt, stay elevated!  By keeping high up by grappling to high ledges you will avoid most of Reinhardt's attacks, including his melee Rocket Hammer swings, Charge ability, and his ultimate Earthshatter.  You will just need to watch out for Fire Strikes, which he can aim in any direction.  You will simply need to strafe around to avoid that.

Soldier 76:  The further away from you are from Soldier 76, the better.  At long ranges, Soldier 76's Pulse Rifle has fall-off distance damage with less accuracy, so he will do much less damage to to you from a distance.  Being further away also means you will be safer from Soldier 76's Helix Rockets, giving you enough time to strafe to avoid it.

Soldier 76's ultimate Tactical Visor is still very deadly to Widowmaker even from long range, as his shots will always connect to a target, and Widowmaker should be one of his prime targets during Tactical Visor.  When you hear Soldier 76 use Tactical Visor, you are better off hiding from it until it is over.  NOTE:  When enemy Soldier 76s initiate Tactical Visor, they will yell "I got you in my sights", while allies won't say anything.

Widowmaker:  Vs another Widowmaker it usually comes down to who spots who first and who has the better sniping aim.  An enemy Widowmaker should also be one of the first targets you should look for before sniping.

Be cautious when the other Widowmaker uses Infra-Sight, as she will know exactly where you are at when popping in view.  In this situation consider sniping other enemies and stay out of sight from her for the time being.

Torbjorn:  Torbjorn's turrets are very powerful at all ranges and can take out Widowmaker if she is not cautious enough.  However, Widowmaker can do some decent sniping damage to Torbjorb's turrets, as one fully charged sniper shot can kill a level one turret, or two fully charged sniper shots to destroy a level two turret.

Widowmaker can have a difficult time actually killing Torbjorns that stay around walls or corners repairing his turret.  Widowmaker's Infra-Sight can help her to predict when a Torbjorn peeks within view while he repairs or build turrets, but overall she will depend on other offensive allies to really deal with Torbjorn.  Infra-Sight also does not work on revealing turrets.

Torbjorn rarely needs to worry about Widowmaker's Venom Mines, since its mostly used closer to her on offensive heroes.  Turrets can attack and destroy Venom Mines as well.