I think it very much depends on which versions of the films you see. The Director's Cut of Alien3 is, in my opinion, absolutely stunning (though they are bastards for killing Newt and Hicks).
I thought it was an excellent psychological horror that took the franchise back to its roots, particularly with the speeches to the inmates and the attempt to corral the Alien, and so on.
I quite liked Alien: Resurrection. Very, very strange that Ripley existed at all, but overall the film isn't that bad really. Yes, some bits are very contrived (such as the hybrid, but in my mind that is no worse than the Alien aboard the drop ship of Aliens), but I did like how it continued the Corporation/military obsession with the Xenomorphs, and the mercs had a lot more character than the Marines of Aliens did (mainly because they managed to exist for longer).
Alien and Aliens are awesome films, I am not disputing that, but I do not feel that the sequels are as weak as is commonly believed.
Had not heard about Prometheus, and I've still not seen Predators. Looks like I have some catching up to do!