I really want to say Meteor Mash, but Bullet punch has its uses as well. It does pretty well as a priority move assuming the pokemon that's getting attacked by it isn't resistant to it. I've learned however, that Meteor mash isn't always dependable, despite the possible attack raise and the huge base power and whatnot, there's always the issue of accuracy.
So with that being said, I choose Bullet Punch. Now how about this...
Leech seed or Giga drain?