Well for me it would have to be in this order:
1. Star Wars IV, V, AND VI..(THEY GO TOGETHER SO DONT EVEN ARGUE...JUST 3EPISODES TO MAKE A SIX HOUR MOVIE)
2. ALIEN(not Aliens 1 like someone wrote it was called Alien and thats how you tell it apart from the rest!....That movie actually scared-the part when he is in the tunnel shaft and cant see anything? damn!)
3. Star Trek 2: The Wrath of Khan(now this is what huge starships battling in space should be like!)
On a last note......and I know i will get razed for this but the "Matrix" was waaaaaaay overrated. Dont get me wrong I have all 3 movies but...it just didnt do it for me...I mean did they watch the MIB credits and come up with that brilliant idea and mix a little neo(no pun intended)buddhism and lord knows what else(dianetics maybe?) to get the sci-fi fan boys in a frenzy?
And that ends my list!