Excluding ultimates, a Widowmaker headshot does the most damage with a single projectile: 120 x 2.5 = 300. This ties with Reinhardt's charge which can do a maximum of 300 damage to a single target (not 350!).
Reaper's shotguns can theoretically hit for 280 damage if every pellet were to headshot, which seems extremely unlikely unless maybe point-blank with a Dva, whose armour would nullify half of the damage anyway, since it works on a per-pellet basis.
So going with a Widowmaker headshot / Reinhardt charge:
- Base damage: 300
- With Mercy's damage boost: 300 x 1.3 = 390
- With Ana's Nano Boost: 390 x 1.5 = 585
- With Zenyatta's Orb of Discord: 585 x 1.5 = 877.5
I'm not sure whether it rounds up or down. I'd be tempted to guess it rounds up.
This is enough to one-shot any character in the game. However, on max HP and with a Nano Boost themselves all of the tanks except Zarya should be able to survive this.
McCree's ult does the most damage at 1020 if charged for the full duration, with Dva's just below at 1000 at the center of its explosion. Dva's can be buffed by Mercy if she is buffed at the moment of the explosion so presumably all other buffs effect it as well. Neither of these can have headshot damage multipliers.
Worth noting, it would be incredibly hard to actually get an exact 6 second charge on your deadeye, because it would require perfect timing and even then you are maximising the amount of time your opponent has to hide or kill you, as well as adding the potential risk of not getting any shots off at all if you wait too long. If you fired more than 0.12 seconds before it maxed out on damage then Dva would be doing more damage with her ult, and from potentially a much safer position.
Nevertheless it is still technically possible:
- Base damage: 1020
- With Mercy's damage boost: 1020 x 1.3 = 1326
- With Ana's Nano Boost: 1326 x 1.5 = 1989
- With Zenyatta's Orb of Discord: 1989 x 1.5 = 2983.5