If you ask me, AvP2 was/is by far the best of the bunch. I thought it was GREAT to see an AvP game that actually had a respectable story. Personally I think the story is solid enough that they could turn it into a movie. I love how the 3 campaigns intertwine with each other, the actions of each protagonist inadvertently aiding the other two protagonists without any of this "Enemy of my enemy is my friend" bullshit.
I also really enjoyed Predator: Concrete Jungle. But I've always been a HUGE Predator fan while only a moderate Aliens fan.
As for the worst game, I really don't think there's a "worst", but if I had to pick one I'd go with the newest one that was released for the consoles but only because the story doesn't fit in as seamlessly as it did in AvP2 for the PC...it gets kinda hard to follow at times. That said, I love the mechanic they made for the Smart Disc in that game, made it really fun and a viable weapon without being cheap and "throw disc, get a kill".