For me, it would have to be GameTrailers because you can download full HD quality trailers for almost any game at free registration. There is also GTD, where you can actually buy stuff with free virtual currency you earn from watching videos, submitting videos, commenting, posting, rating, blogging, etc. Stuff you can buy ranges from upgrading your profile to a next gen console game like Resistance to the mighty PS3 itself.
GameTrailers Registration Link - CLICK