Harder question than the Terminator one.
Alien 3 and 4 were substandard movies. 3 was okay, but 4 just stunk.
However, the first two are excellent.
Alien has a better story in my opinion, but a plot that depends on suspense means that it moves a bit more slowly.
Aliens was just so cool in every possible way, and barely wins this one for me.
Barada