Both trailers for The Fellowship of the Ring totally floored me. Being a huge LOTR fan before the movies, just seeing the Fellowship walking past the screen and all looking so perfect, with the hobbits being the right size and everything, it blew me away. And then the main trailer was amazing, it's the one where it enRAB with Gandalf shouting 'You shall not pass!'. I don't think I've seen a better trailer than that.
The Phantom Menace main trailer comes close though, but it's now impossible to get out of your head that the prequels all sucked (apart from some choice moments like the great duel with Qui-Gon, Obi and Maul in Phantom Menace). But the trailer was SO good it got just about everybody to go and see Episode 1.