The Phantom Menace - probably the best example of a trailer being a lot better than the movie.
Spider-man 2 - just looked like a Spider-man comic brought to life, so fast and full of action. For once the actual movie had an action scene that of the quality the trailer promised.
Batman 89 trailer - I had this split up on a BBC doc that was on tv a few months before the movie came out, I just kept watching the parts over and over, could not believe how good it looked . 'Lieutenant is there a 6 foot Bat in Gotham city?'
X-Men - I had a recording of the 2nd half of this(the part that uses the exciting final confrontation music from 'Dark City') off tv and watched it so many times, probably my most watched piece of footage ever. Before seeing this I was preparing myself for an X-Men movie that got a couple of things right, this looked like it got it totally right, and it did.
Also has added bonus of seeing an angle of Logan's face as he cuts through the shotgun that was not in the movie.
Attack of the Clones - Not as exciting as Phantom Menace as now I knew that it was possible to mess up a SW film. But, this got me really excited seeing the footage of Jango Fett. I was disapointed that we didn't get the little bit of extra footage shown here of the Obi-Wan/Jango fight, where Obi-Wan is spinning around while deflecting laser blasts, that was my fav moment of the trailer.
Star Wars prequels always had amazing trailers.
edit: Also, the Japanese Watchmen trailer that had no dialoge and used the Philip Glass music. Watched that a lot too.