I think the Alien movies have been the most consistently good, with the exception of the 4th film. Also, I think Alien itself is a perfect movie so that series wins out.
Terminator is a close second, but I'm just not as gaga over T2 as a lot of others seem to be. I do like it, even more so than I used to, but T1 is still my favorite of the series. T3 was terrible though and TS might have been able to redeem, but ended-up being more of a forgettable film.
Predator and Predator 2 are great films, but to me (with the exception of the creature design and characters like Dutch, etc.) just not in the same league.
And AVP not surprisingly finishes last, just filler, but I still thought the 2nd one was at least fun... as long as you didn't mind the complete mindlessness of it all.