To go by individual film,
Alien
Best - Ripley
Worst- I think Brett would fill this role, I don't dislike him at all, all the characters are memorable and I like them all but in terms of importance, dialogue etc Brett I think might be the least developed. I supposed I could Jonesy as his existence kills Brett and nearly gets Ripley killed too
Aliens.
Best- It is really is hard for me to choose here because Ripley really shines in this movie but the Marines are so memorable as well. Husdon, Hicks and Vasquez. Hell even Bishop.
Worst - I can't really think of one unless I including character behavior that I personally find questionable. Anne Jordon maybe...for not helping her husband. While in the film we don't know exactly how Russ is attacked or how Anne reacted but the novels seems to show that she didn't do much to help him. she doesn't try and help him pull the facehugger off even if is likely futile and she leaves him out in a stormy weather for hours, she covers him up but still. Mind you if I was going to go by novels, I would say the scientists/medical personnel. They did absolutely nothing to help the infected colonists and their logic and excuse for doing so was poor.
Alien 3.
Best - Ripley/Clemens/Dillion. Interesting arc for Ripley, Clemens was sympathetic and Dillion was just brilliant, he goes up against an Alien just to buy Ripley some time. He lasted longer than Parker.
Worst- There is a few background characters that don't really add much.
Alien Resurrection.
Best- Ripley 8/Johner/Elgyn.
Wost- Wren, some scientists and leaders. They spent years trying to get Aliens and seem to know enough about them to know they require hosts, yet they don't bother building acid proof cells and keeping them seperate. Not to mention that the organization here is just seriously immoral. Kidnapping innocent humans instead of simply using animals or at worst death row inmates.
Prometheus.
Best - Shaw/David
Worst- you know them, those bad bad scientists.
Alien Covenent.
Best-Walter. I find him quite interesting and similar to bishop.
Worst - David, he has just gone too over the top in villiany, there is no nuance with him anymore, and meta wise he might be responsible for the poor choices the franchise has taken such as more focus on AI intead of the Alien.
Predator.
Best- Dutch/Billy/Mac but really all of them shine.
Worst - Honestly I can't think of one at the moment, the characters are memorable and larger than life.
Predator 2
Best-Jerry/Harrigan
Worst- Tony Pope. I hate paparazzis
Obviously he is meant to be hated so not sure if that
counts.In seriousness I would say Danny, as he pretty much got himself killed.
Predators
Best-Stans/isabelle/Nikolai/Hanzo Stans is funny, Isabelle is badass, Nikolai is heroic and Hanzo had a memorable scene.
Worst- Cuchillo. I like Dany Trejo but his character is just not given much to do.
The Predator.
Best-Baxley
Worst-Upgrade/predator dogs.
AVP.
Best-Charles Weyland but I think Max, Rousseau and even Lex was interesting and could have had more to do.
Worst- the cannon fodders, too many characters that are just there to die. If I had to name a major character though it would be Miller, as he almost immediately fails to follow the rules that Lex gives out, he even wastes ammo on a dead hugger.
AVP-2
Best-Wolf but only because of his design, he was utterly incompetent.
Worst-Predalien, the aliens and most of everyone.
