Alien is the best in terms of genre-influencing originality in set & costume design and script.  Aliens is the best in terms of fun action and expanding the mythos. Resurrection met my expectations; it is interesting, but nothing mind-blowing (but I hate the newborn). It does the best job of exploring the evil of the franchise’s main villain, The Company, albeit in a somewhat melodramatic way.

Alien³ is the only movie that I would say disappointed me, in that it seemed to fall short of its potential. I wish it had more positive continuity with Aliens, and had developed the mythos more. 

I won’t even comment on AVP and AVP:R; they’re like the new Star Trek movie in that they are just regular action flicks that attempt to bring in extra box-office dollars by utilizing characters that have an existing fan-base, but other than that have no relation to the franchise.  Still, they have aliens and predators and so I watch them!

Thanks for the question!