I think predator is arguably arnold's best movie - so if you're only gonna watch one arnold movie you should watch that one - definitely entertaining even if you're not a big arnold fan
I vote the first two Terminators as his best. Sci-fi classics in my view. Particularly the second which I still think is James Cameron's best film to date. But, I'd put Predator right behind it. The Mexican jungle setting adds so much atmosphere and I think the Predator is one of the great movie monsters. He's one ugly muthafucka!