It would depend on the Pokemon receiving the attacks, however here I put the calculations on a random custom opponent and let's see the results!
Terrakion
252+ Atk Choice Band Terrakion Close Combat vs. 0 HP / 0 Def (custom): 644-758 (188.85 - 222.28%) -- guaranteed OHKO
As you can see it's 188.85 - 222.28% damage output.
Rampardos
252+ Atk Choice Band Rampardos Stone Edge vs. 0 HP / 0 Def (custom): 322-379 (94.42 - 111.14%) -- 62.5% chance to OHKO
So here it's 94.42 - 111.14% damage output.
Darmanitan
252+ Atk Choice Band Sheer Force Darmanitan Flare Blitz vs. 0 HP / 0 Def (custom): 442-522 (129.61 - 153.07%) -- guaranteed OHKO
As you can see here it's 129.61 - 153.07% damage output.
252+ Atk Life Orb Sheer Force Darmanitan Flare Blitz vs. 0 HP / 0 Def (custom): 383-452 (112.31 - 132.55%) -- guaranteed OHKO
Here it's the Life Orb + Sheer Force calc. 112.31 - 132.55% damage output.
__
So in the end Terrakion is the most powerful.
Btw, I am using: http://pokemonshowdown.com/damagecalc/