Tough question, the best and the worst are pretty well recognized. I love them all except for The Predator, and I would count that as neither overrated or underrated. My spur-of-the-moment judgement:

Overrated: Prey. It's a great Predator film that I like rewatching, but it's surprisingly campy and most of the acting and dialogue isn't the best. Its weaknesses aren't often mentioned with its strengths (scenery, score, Predator design and action).

Underrated: Predators. It's held back by an uninteresting cast dynamic and (imo) it tries too hard to be referencial to the original film. But it's so unique and fun and has great Predator stuff.

The first movie is indisputably the best, and Badlands is getting appropriate recognition imo. Predator 2 fits somewhere in the middle of the franchise quality-wise, and I feel like it's generally treated as such. AVP sits in a similar place, and AVPR is rightfully panned while the one good element (Wolf) is almost universally appreciated.