Given the correct situations, it's actually Rollout
A level 100 Shuckle can potentially deal the most damage in one single attack through the use of numerous stat boosters; by using Helping Hand by an ally, holding a Metronome, Power Trick, a Skill Swap to Pure Power, and 6 Attack boosts. Also, Shuckle's partner must have the Ability Flower Gift and the weather must be sunny. On the 5th turn of using Rollout consecutively without any misses, if used against a level 1 Ledyba, Yanma or Combee with minimum Defense stats, that have been hit with negative Defense modifiers (such as Screech), it can deal 48,121,381 damage with a critical hit.
The base power of Rollout is 480 on the fifth turn, the highest base power of any move as of Generation V. In earlier generations, the power of Explosion was effectively 500, though Generation V changed it by no longer halving Defense.
When used with Defense Curl, however, Rollout's base power is 960.