I've looked around for the answer and ive realized that the most damage would be with this:
*Deoxys(Att), 6 perfect IVs, 252 SpA/252 Att/6 Spe, any nature that boosts an attack and doesnt lower the other. (Lonely,Naughty, Brave, Modest, Bashful, Quiet).*
The damage you would deal with a fully charged ball would be, as dOkTorR (ツ) d00D? said, around 1600 HP.
I couldnt find what exactly the most damage is, but I think it's around 1650.
Hope this helped!
i've come to this conclusion because Deoxys-Att has the highest Attack and Special Attack stats from any non-mega Pokemon, both at 180. (megas can't be used in ST, ofc)