As of now, a Kyogre Water Spout in rain with full HP on a crit is the most powerful attack in the game.
This may change when ORAS comes out and Kyogre gets a Mega Evolution, as its special attack may increase to more than 1.5x what it regularly is.
Here's a calculation:
252+ SpA Kyogre Water Spout (150 BP) vs. 0 HP / 0- SpD Numel in Rain on a critical hit: 158332-186280
So 186280 is the most damage its attack can do.
If you think weather is a boost and shouldn't be calculated, here's another calculation:
252+ SpA Kyogre Water Spout (150 BP) vs. 0 HP / 0- SpD Numel on a critical hit: 105556-124188
So without weather, it's 124188.
Hope I helped!
(This is the damage calculator I used)