Love the thread! I would have to say it's between Giga Impact, Rock Wrecker, Explosion, Sacred Fire, Judgment, Sketch, Outrage, Trick Room and Baton Pass. Lol, long list, but I can explain.
I love Giga Impact and Rock Wrecker just because I love a physical-type Hyper Beam. Also, Giga Impact is just perfect for my lovely, lovely Slaking.
Explosion is nice because its power is 250, and after the Defense halving, it's 500!!! So powerful. Neeeeeed a Life Orb Explosion Lickilicky!!!
Sacred fire is the best because a physical Fire-type move is extreeeeeeemely hard to come by. And a 50% chance of burning!!! Crazy! Also, Ho-oh is awesome.
Judgment. Come on, do I really need to explain?
Sketch is what makes Smeargle what it is. If only it had good stats to match it...
Outrage. More powerful than the crappy Petal Dance and Thrash, and... one type to be resistant to it! WHOO!
Trick Room. Totally reverse the odds between a Jolteon and a Snorlax!!!
Baton Pass. Well come on, hand-me-downs! It's the greatest!
Gun to my head, I'd say Baton Pass. Gotta love raised stats. And Substitute. And Aqua Ring.