The only one truly unwatchable is AVP:R. P2 and AVP get unfairly maligned IMO. Predators, wasn't bad, harkened back to the feel of part 1 a lot, it was just nihilistic which can be good or bad depending on personal taste.
Actually, if I had to really rank them, I'd put 2 above 1 but just barely. I always like the fact Harrigan was just a regular cop fighting this towering alien, it seemed more a challenge for him than Dutch.
So for me:
P2
P1
AVP
Predators
AVP:R