There are so many great designs for predators, it's hard for me to choose a favorite but if I had to, (I feel so dirty posting this) I'd have to go with the Berzerker predator. Mr. Black or whatever it's known as. It's by far the most menacing in my opinion. The lower jaw on the mask makes him look mean and nasty.
Celtic and the Falconer also are some favorites of mine, but the original one/classic and scar are so great as well... but then I can't ignore the city hunter and Wolf. They're all so great.
The only predator I really didn't like from the movies was Chopper. Other notable Predators from other pieces of work that I disliked were Big Red and the ancestral mask from the 2010 avp game