Depends on the situation... That's like asking what's your favorite food. You might love prime rib, but prime rib every day becomes lame and you'd want a chicken instead.
So, the movie I like the most usually is "Aliens". But, I also like "Colossus: The Forbin Project" and "2001: A Space Odyssey". However, if I go romantic, then "Somewhere in Time", or "The Time Traveler's Wife", or "The Adjustment Bureau".