Okay, it may be a big coincidence, but I also like earthquake. Strong move, and it's great when you're in a double battle, as long as your other pokemon has either fainted, or has levitate, or is a flying type (and the move Gravity hasn't been used, or your Pokemon isn't holding the Iron Ball). And I can kill all the rock and steel Pokemon, and be almost 100% sure that it'll hit, thanks to its high accuracy rate. And the hardest type for me to kill? Rock and steel for some reason.
As for the runner-up... I'll have to say Crunch. Why? Because I have no dark-type Pokemon on my team, so the only way I can defeat Ghost-types and Psychic types is with the awesome move... crunch. Plus the fact that most, as in most, pure-psychic types have horrid defence, so crunch pwns at times like those. And crunch is actually quite strong. So I like it.