The first two Alien movies were great; after that they were just milking the franchise.
Serenity is one of the great scifi movies of all time. Creepy, and will have you on the edge of your seat. But you need to have seen the 15 episode show Firefly first (which is great also) to even understand what is going on.
Dark City was an instant classic.
The Matrix is the best of the three movies. The other two are pretty good, if you get caught up in the series, but the first was best.
The original Day the Earth Stood Still is an old classic. And looks even better compared to the Keanu Reeves PoS.
