I'd say each film is better than each other in their own unique ways. Here's what I think:
Alien - The scariest, had the best simple plot
Aliens - The most exiting
Alien3 - The best acting, most atmospheric
Alien Resurerction - The most entertaining
Predator - Great suspence
Predator 2 - Great creature work
AVP: Alien vs. Predator - ?
AVPR: Aliens vs. Predator Requiem - Looks set to be the most entertaining, better creature work