That is a tough choice since it depends on my mood. I love to laugh so I would be most likely to put comedy at the top but a good action or drama is great too. I really prefer movies that have have many elements:
Action-comedy:
Rush Hour 1 & 2, 5th Element, Independance Day, etc...
Certain romantic comedies are good too:
Overboard, Groundhog Day, Roxanne, etc....
I love anything Historical too:
Gladiator, Saving Private Ryan, The Patriot, Dances With Wolves, etc...