Aliens is without a doubt my favourite film of all time, strangely enough I just watched it again the other day in HD.
The thing about it is, no matter how many times I watch it I always get to the bit where they're trying to escape through the air ducts and Vasquez & Goreman set off the grenade and think, "Just hang on Newt, come on! Hang on! Nooooooo!" Not many films have me rooting for an alternative plot that I know isn't going to happen even though I've seen it a dozen times.