Favorites in order: Flusher, Falconer, Berserker
Flusher was my favorite for his tusk mask and the fact that he was the dog handler; Falconer also had a very cool mask design. Berserker of course was pretty awesome in the movie, and put up a good fight through it.
I didn't count Classic since he defines the Predators, and will always be awesome since hes the original
.
Edit: Favorites are subject to change; after I saw the movie I've begun to start liking Berserker a lot more
.