1 was very good, really dark and had a very "tech noir" feel about it, quite sinister and made you feel on edge no matter how many times you watch it!
2 was the best of the lot for me. As has been said was year ahead of its time! (like most of cameron's films Avatar, The abyss, Aliens ect) also you could watch it not having see the 1st and not feel like you have missed out.
3 was utter garbage! the story was wasted on the director! It could of been soooo much more but failed!!
4 was reasonably good the only critisism was the way Kyle reece was portrayed.