I agree with the comment above me. Avatar was just an amazing movie. May be some better ones out there, but this one is very recent so none come to mind.
Please consider following the guidelines of these forums. One of these guidelines is writing and explanation on your choice, so you actually have more than just "I like *movie title here*", because frankly, that is spam.
I currently have no favourite movie, as I find it weird to compare different genres with each other. However "How to train your dragon" showed me that Dreamworks are doing a really good job at animation movies, if not better than Disney, along with telling a good storie in an interesting way.
Monty Python and the Holy Grail. Can't believe no one has said that yet! Others on my list include The Princess Bride (It's NOT a chick flick), Armageddon, Planes, Trains and Automobiles, The Song Remains the Same, The Rookie and 48 Hrs. Hoping to see Forrest Gump soon.