Predator 2.
I see a lot of people are saying he was weak because Glover killed him. You do realize that Glover had to shoot him 7 times with a 55. caliber shotgun, cut off his forearm and have him get thrown through a solid brick wall in order to defeat him, right? I'd say he was a lot tougher than the first one, who went down just because a log fell on him.
That and from what we've seen, the P2 Predator was quite proficient in melee combat, whereas the first relied on stealth and his caster, which probably wouldn't work if he had to go against another Predator.