It's hard to pick one, so here are a few in no order.
Pulp Fiction
Great screenplay, wonderful dialog and direction, the acting wasn't top notch but the style and presentation of the film make up for it, plus it was always suppose to be a B movie.
The Empire Strikes Back
The best of the Star Wars trilogy because Lucas chose Irving Kersner to direct it, his excellent direction focused more on the characters then the 'epic' struggle making them feel real and engaging.
Memento
I'm a sucker for a good artistic film, expecially if it is shot in an interesting fashon and cut that way as well. Christopher Nolan put together something special in the way the movie progressed backwards, and how that worked in with the theme of the film.
Vertigo
Hitchcock was the best, and Vertigo was his best and most chilling film. Less horror more thriller, and the acting by Jimmy Stewert and the supporting cast was amazing, the depths of depression and obsession explored in Vertigo are just amazing.
