Easy:
Apocalypse Now + Aliens + Red Dawn + Starship Troopers = The most awesome intro in gaming history.
As well as:
Meanwhile that, my friends, is pure style you just witnessed. Not only being fun to watch, the Thief intro just sets the mood and style for the rest of the game to come as well as giving a little glimpse into the backstory and even a little foreshadowing. You just don't see style like that anymore!
Although serious props go to Bioware for the Mass Effect 2 'intro sequence'. I was careful to avoid any spoilers before the game was released and had just finished a run through of the original Mass Effect beforehand, so the intro to Mass Effect 2 was... amazing, to say the least! I was in all honesty blown away and left speechless by those first few opening minutes. Damned impressive!