IFC lists the 50 greatest trailers of all time. Trailers are like episodes for Law & Order for me -- ten minutes after viewing and I can't remember a thing about them -- so I don't really have any favorites, but this list seems like a solid collection.
Update: They also polled a number of experts to weigh in on their favorites. The article led me to the Golden Trailer Awards, an annual awards show for the best movie trailers and posters. This year's winner was the trailer for Star Trek (I'm guessing it's trailer 1).