Basically, what is the most amount of damage any Pokemon could do to another without stat altering moves.
This is extremely hard to tell. Very. But I believe the answer is a Heatran using Eruption in the Sun against a Paras.
252SpAtk Choice Specs Flash Fire Heatran (+SAtk) crit Eruption in Sun vs 0HP/0SpDef Dry Skin lvl 1 Paras (-SpDef): 3876109% - 4560218% (426372 - 501624 HP). Guaranteed OHKO.