1. Bastion: Turret Mode is the ONLY Mode

Bastion is a much more nuanced, complex hero choice than he
may initially appear - sure, his turret mode makes him a pretty stellar
choice for defense missions, but if that's the only time you're picking
this bird-lovin' robot, you're making a huge mistake.
When To Use Him: Bastion is best utilized
in Attack Escort missions, obviously. While the rest of your team
huddles around the objective, your best move is to plant yourself way
behind everyone else in turret mode and wait patiently for someone to
come into view (or until you get sniped). Really, you should never be exiting turret mode - your DPS is much higher, after all. And, realistically, you should be shooting your turret NON-STOP
(since your ammo-per-reload is so huge, there's no need to hold back).
You never know when an opponent might pop into view and straight into
your stream of bullets. Sure, you'll draw a little attention to yourself
that could get you sniped or chain-pulled or rocket-ed, but the payoff
is worth the risk.
He's also best utilized in King of the Hill missions, and also Defense Escort missions. Find a nice spot where you are totally exposed from every angle (so you'll have perfect visibility and potential to kill things all around you), get in turret mode, and let 'er rip!
2. Widowmaker: Why Your Team Needs At Least 2

The game's most prototypical sniper can be deadly with her one-shot kills and opponent-position-exposing ultimate, but her low health has made some people afraid to utilize her to her full potential.
When To Use Her: Every single mission, every team needs (AT MINIMUM) 2 Widowmakers. It doesn't matter if it's Attack Escort - if your team is at only 1 (or, more terrifyingly, ZERO) Widowmaker, it's your duty to also pick her. Ignore your teammates screaming at you and frantically typing "NO WE NEED A HEALER" - you were born to snipe. Will you die a lot? Yes. Will you almost never contribute to moving the objective forward? Absolutely. Will you miss most of your sniper shots because the game is too frantic and you're playing with a controller? Unquestionably.
But every now and then you'll get a headshot on Roadhog that will totally almost kill him but then he'll heal and you'll be able to say "well, that's bullshit."
When To Use Her: Every single mission, every team needs (AT MINIMUM) 2 Widowmakers. It doesn't matter if it's Attack Escort - if your team is at only 1 (or, more terrifyingly, ZERO) Widowmaker, it's your duty to also pick her. Ignore your teammates screaming at you and frantically typing "NO WE NEED A HEALER" - you were born to snipe. Will you die a lot? Yes. Will you almost never contribute to moving the objective forward? Absolutely. Will you miss most of your sniper shots because the game is too frantic and you're playing with a controller? Unquestionably.
But every now and then you'll get a headshot on Roadhog that will totally almost kill him but then he'll heal and you'll be able to say "well, that's bullshit."
3. Mercy: Heal Other Healers

Overwatch's most reliable healer is the angelic Mercy, who
has a healing beam, an attack buff beam, and the totally boss ultimate
that resurrects fallen teammates.
When To Use Her: Preferably, when there's
already at least one healer already on your team. Sure, you guys already
have a Lucio - but he's too slow of a healer, and he doesn't draw
enough attention to the other team as to who's being healed. That's why
you gotta go with Mercy - double the healers, double the health for your team.
Make sure you just focus your healing beam on one hero the entire
match, turning them basically invincible (we recommend finding a Tracer
or Widowmaker). But to REALLY show 'em you're a healer pro, find your team's other healer (Lucio or Zenyatta or ANOTHER Mercy) and focus your healing beam on them the whole match. Then it becomes a human centipede of healing power, and your team is unbeatable!
Note: one pretty stellar alternate
strategy is to use Mercy's pistol exclusively instead of her healing
ability - whipping out that pistol and firing away can be a devastating
strategy, since no one will expect your healer to be dealing out damage!
4. Reinhardt: Put Away the Shield

The biggest, tankiest hero in the game is Reinhardt - with
his huge health and massive energy shield, he's practically a necessity
in any Attack Escort mission. But if that's the only use you have for
him, you're missing out on a lot of his true potential.
When To Use Him: Sure, Reinhardt's energy
shield is nice - it deflects bullets and explosives, and your teammates
have the ability to shoot through it while it provides protection for
them. But here's the thing: you can't attack when your shield is up, and
that means you can't get any awesome kills to get that Play of the Game
moment. So your best move is to NEVER USE YOUR SHIELD.
Instead, you need to find a cluster of enemies, and immediately do your
boost-tackle towards them. If you're lucky, pinning them will get you a
kill (lower health heroes like Widowmaker and Tracer will go down
instantly) and in the confusion you can use your hammer to rack up some
serious KOs. They gave Reinhardt a giant hammer for a reason - to use
it!
5. Tracer: Stay Still and Stay Deadly

Overwatch's feisty mascot is quick and deadly - but her low
health leaves her susceptible to pretty quick deaths if you're not
careful. She requires a pro's touch to truly use her in a way that
really makes her an asset.
When To Use Her: Tracer's speed and
teleporting abilities are her greatest assets - and her greatest
liabilities. The fact is, no one can really tell where she's going to
end up or what's even going on when using her. She just moves too fast
for even the person controlling her to keep track (I often get violently
nauseous while playing her, and also most non-stationary characters)!
So when playing as Tracer, you need to slow it down and come to a full stop as often as possible. This will allow you to more carefully aim your double-pistols and make Tracer into a truly deadly foe.
6. Torbjorn: The Ideal Turret Placement

The closest equivalent to Team Fortress 2's Engineer is
Torbjorn - the dwarven weapons-maker who specializes in turrets and his
rivet gun. Naturally, you'd think he'd primarily work as a defensive
character - but you'd be dead wrong.
When To Use Him: Torbjorn is one of the
most effective and versatile heroes in the game, thanks to his turret
placement ability allowing him to essentially be in two places at once.
The main question when playing as Torbjorn is - where should you place your turret? You don't want to be constantly moving it around, after all. And the answer is simple: right next to the opposing team's spawn point.
There's no better way to throw the other team off their guard by
placing the turret immediately outside their base, so they're being shot
at the instant they walk out. Will they destroy the turret immediately
(and likely kill you too, given this is where they're the strongest and
most focused in numbers)? Yes. Will this leave you to respawn on the
opposite side of the map, taking a huge amount of time just to get back
to the objective? Absolutely. But you'll be able to slightly annoy a few
of your enemies for a second or two, which will put them on tilt the
rest of the game (which will last approximately 4 minutes, since they'll
win in record time).
7. Lucio: Speed's the Name of the Game

Lucio is a healer, sure - but he's so much more. His
weapon's secondary function allows him to push away opponents, and he
can alternate his primary buff from health to speed. And therein lies
Lucio's true strength.
When To Use Him: Again, Lucio's buff has
two modes: refilling health and buffing speed, but only one at a time.
And while many see Lucio's primary duty on the team to be a healer,
these people are being short-sighted. Basically, if you're playing as
Lucio, you should NEVER switch away from the speed buff
- as that's where Lucio really shines (his heal is - simply put - too
slow). Keep the rest of your team constantly moving fast (except anyone
playing as Tracer, who should remain stationary and unmoving as often as
possible). The opposing team won't be able to hit anything with your
whole team zooming around like a Benny Hill sketch. Especially if your
team has a Mercy (who should be spending the whole match healing and
protecting you), victory is assured.
8. Reaper: The Unexpected Sniper

One of the deadliest characters in the game, Reaper lives
up to his alias - he's a bringer of death and destruction, and in the
right hands, he's virtually unstoppable. But how do you make your hands
the right ones? We'll explain...
When To Use Him: Reaper's biggest advantage is that he doesn't have to reload his guns, ever. See, instead of putting more ammo into his guns, he literally just drops 'em on the ground and pulls out BRAND NEW ONES!
Think about that - that's badass as hell, but only if you know what
you're doing. The key is to take advantage of this as much as possible -
hit the reload button after every single shot you take, so that you can
continuously be pulling out new sets of guns. This will absolutely
terrify your enemies.
The next thing you need to remember about Reaper is his teleportation ability.
This allows him to get to high spots that would otherwise only be
accessible to the higher-flying characters like Pharah - but no one's
going to expect to be shot from afar by Reaper. Get to the highest, most far-off position you can find, and start sniping away at your enemies - they'll never know what hit them!
...of course, if you're going to pick a sniper, you should
probably go with Widowmaker (if you can convince your team to go for the
ultimate 6 Widowmaker lineup - aka "The Sniper's Gambit" - all the
better)