I like Steve Martin and he is also an actor. So in that sense he is my favorite actor. But not necessarily who I enjoy watching as an actor.
Clint Eastwood is my favorite Western actor and is probably my favorite. Jack Nicholson perhaps is the most interesting and is probably my favorite by default.
I like roles perhaps and the actors who made them. I like Tom Hanks a lot and his role in Forrest Gump despite parodied is fantastic IMO. Christopher Walken in the Deer Hunter is stellar but there aren't a lot of other roles I like him as much in. Woody Allen is a favorite. I can happily re-watch his movies but I think I like him more as a movie maker.
Have to choose one? Jack Nicholson