The original by far. Predator 2, for me anyway, suffers from it's very specific timeline setting-Los Angeles in 1997 looked nothing like that, obviously.
Danny Glover is a good actor, but I think he was miscast here, and I tend to find the whole idea of some cop defeating an alien from another star system to be a little impossible. They'd have been better off focusing on the CIA guys trying to capture the beast.
Also, I really dug the way Arnold sorta devolves in the original-he, and his team, go from attempting to kill the Predator with guns, and then a nasty, nasty ambush, and then finally Arnie's covering himself in mud to avoid being seen on infrared and making spears out of knives and stuff. There wasn't an equivalent to that in the 2nd one and it was really rather disappointing.
Plus the ridiculously OTT "PREDATOR 2" in gigantic neon letters kinda aggravates me from the start.