@Mist already posted an Eve trailer, but this one is my favourite, even if old. Maybe because when I saw it first it was at a Fanfest on a giant screen.
The Gears of War trailer was pretty brilliant. In an age where the more loud and over the top your game was, your trailer had to exceed even those levels... they dialed it back in the opposite direction. Personally, I think the game series is pretty 'meh', but this trailer hyped the game to such a degree that it was the system seller for the early Xbox 360 days.
I like all the GoW trailers a lot, the games are OK but most of the trailers have always had that just right tone (that escapes the games themselves). Some of my most rewatched though, aside from the Halo 'Believe' one, are these Transformers ones. Great music, great shots, also love the games.