METRONOME!! It may not be the best move to use, but it's certainly the most fun. :) In SoulSilver, I actually made it a personal challenge to level-grind my Togetic using only Metronome, and I used it quite a bit in regular battles as well; I kept a log on my computer's notepad program listing all the moves he called forth. One of my fondest Pokememories...
Runners-up include:
*Assist - the same random fun of Metronome, but a bit more reliable - it's so cute seeing my other Pokemon lending their techniques to my little Skitty. :3
*Super Fang - halving the opponent's HP regardless of level or stats can be pretty OP, at least in-game, and I'm pretty sure Lars the Watchog used it against every boss in Black 2.
*Ghost-Type Curse - a signature technique of my favorite Type, and another staple of my Black 2 run.
*Beat Up - the dark and brutal power of friendship! `*~*\(^w^)/*~*`It's actually moderately strong, too, once you get 6 Pokemon.
*Skill Swap - I hardly ever end up using this move, but whenever my Pokemon tries to learn it I'm like, "!!!! Yes!! I want Skill Swap!!!!" I'm not sure why...Also, it does some interesting things to the game mechanics.
*Ice Beam - I love the Ice-Type, and this move is its most iconic, one of its most reliably powerful, and just plain the iciest!
And, a bonus - my favorite Z-moves!
*Breakneck Blitz - no reason other than I used it a whole lot in Moon
*Subzero Slammer - ICE <3 Doesn't this move have the best name, too? I feel like I could blow out a cloud of breath just reading it!
*Oceanic Operetta - I'm so looking forward to using this in Ultra Moon - that animation is seriously one of the best in the game
*Catastropika - Ridiculous in the best way - you just straight-up hurl your Pikachu in the air! (I'm still kinda secretly hoping it'll show up in the anime...)
*Let's Snuggle Forever - no explanation needed.
...I really like moves. <3