I'd have to say The Day After Tomorrow and 2012 are two of my favorites, but honestly, I have a really tough time picking a single best in my mind. I also enjoy Up and Kung Fu Panda. :P
I agree with emolovesblack, The Princess Bride (such a misnomer because it's really epic) is a great movie. Here's some of my favorite scenes:
The following swordfight did not use stunt doubles (other than for the flips and such), the actors had to learn to fence with each hand. Cary Elwes used these skills in later films like Men In Tights.