There was a list I saw somewhere at somepoint, but as some basics for the hardest heroes to deal with
Rein: Symetra (rclick goes through his shield), Winston (weapon goes through shield), and Mcree to a lesser degree (Flash bang above his shield to drop it)
Widow: Another widow, Dva or Winston since they are mobile (they lack the damage to kill her usually, though...she just runs away most times)
Tracer: Mcree (flash n' fan), Roadhog (hook n' shotgun), Junkrat to a lesser degree (steel trap stops her)
Bastion: Genji (reflect), Zarya (bubbles for energy when bastion shoots, then rclick energy bombs), any good ranged that knows how to sneak n' peek (widow, hanzo, pharah)
Toblerone: Same as Bastion. Dva is also good vs. him.
Mcree: Roadhog (hook n' shotgun), any good ranged