Not counting a ramp up, Doom and Claire have the highest attack rating, so it is probably then. But, Wasp is close and has a higher crit rating, so it most likely is her is she crits.
Guillotine 2099 at 600+ Combo, I suppose. She should deal over 30'000 energy damage per every hit, so she makes a 150'000 damage with a 5-hit combo. Now, maybe an incredible lucky Cosmic Ghost Rider that makes 5 critical hits in a row... maybe he deals more damage. I have no other champion in mind who is able to make so much damage
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Or king Groot with his permanent armor breaks
Doom and Clair
If we're talking ramp up, I'd say Hela, or anyone with infinite damage ramp up.
Now, maybe an incredible lucky Cosmic Ghost Rider that makes 5 critical hits in a row... maybe he deals more damage. I have no other champion in mind who is able to make so much damage