I voted for Earthquake, because it's the only move that makes me like Ground and Rock types even a bit. Also, I kind of love it a lot, since it has saved me in many situations... Although I really don't understand how an earthquake can miss its target.
That aside, my ultimate favourite move is actually Volt Tackle. Electric type moves are just the coolest and Volt Tackle is just... well, it just is my favourite move. I also like Dark type moves, though. And Fire Blast. I'm very attached to that move for various reasons.