I think that its just becouse its on, but Saving Private Ryan's D-Day opening scene has to be easly top 5. I kinda want to kill some Germans Nazi scum right about now.
Goodfellas comes to mind where it opens with the 3 guys opening the trunk to find the guy they're burying alive. Then it goes into how Henry Hill became a gangster. Love it. Gladiator has a pretty sweet opening too.
Saving Private Ryan...holy crap, I remember being in the movie theater with my jaw on the floor that entire scene
It's not a single scene, but the first 30 or so minutes of Full Metal Jacket (right up to where Pyle kills himself and Hartman) is like one of the best segments of any movie. I only bring it up because thinking of 'Saving Private Ryan's' first scene reminded me of war movies in general. As for opening scenes, 'The Dark Knight' had one of the better ones in recent memory. I've always loved the opening scene of 'Pulp Fiction' with Samuel L. at his awesomest. The remade 'Dawn of the Dead' was alright, but it's opening scene with the little girl eating the protag's boyfriend and her trying to escape with zombies running at full speed all around her was crazy. 'Quantum of Solace' had an action-packed opening.
The opening 30 minutes or so of FMJ is brilliant. It's easily the best part of the movie, and quite hilarious as well. As for best opening scenes, I've gotta think The Dark Knight is right up there. V For Vendetta has a pretty good one as well, with V blowing up those buildings to start the movie.