Best hero from each class and why?

Offense: Tracer - Annoying as shit, easily apply pressure in many situations.
Defense: Mei - I love her on D, effective walls and annoying headshots .
Tank:Zarya - I love her for how offensive she can be but very tank at the same time.
Support:Lucio - Battle medic, love his kit.

 

Offense: Pharah. A direct hit from her rocket launcher at point blank range is virtually a one-shot kill. Her ultimate is also extremely good when paired with Zarya's ult.

Defense: Mei. If you can catch the entire team with your ult at the CP and your team can follow up, it's a guaranteed team kill.

Tank: Reinhardt any day. He is by far the most versatile or key piece for a team. (Although I'd pick Winston since he is so darn good with eliminating the supports)

Support: ZENYATTA. Hurts like a bitch if you can land your balls with Discord. Ultimate will save your entire team too, assuming they weren't tagged by Ana's E. I switched from maining Mercy to Zenyatta simply because of the fact that I'm contributing considerably more than just being a mere healer.

 

Offense:
mccree, if you can aim, you can 1v6, good damage close range/long range, stun, great 1v1er

Defense:
mei for sure, off-tank, wall, ulti, great for most scenarios.

Tank:

reinhardt - block, damage, charge, all around great champion for most scenarios,

Support:

Lucio - aoe healing, mobility, hard to kill, engages/disengages fights, disengages ultis (reaper, etc.)