This is a really tough decision since they're both pretty damn good. 'Predator' is just a quintessential Arnie classic, while 'Predator 2' simply builds upon what was set up by the titular character to great effect.
Ultimately, though, I'd have to go with the original. Mainly because of the final confrontation between Arnholt and the Predator (kicked off by one of his greatest ever lines
)
But that's where the sequel stalls for me. The climax ends up dragging on a bit too much and the creature is eventually defeated in an anticlimactic manner (in my opinion).
But yeah, both films rule anyway!
