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