You know what I've realized lately? That an actor is often only as good as the part. Take Tom Cruise. He is almost always cast as "Tom Cruise in: Another Tom Cruise Movie!!#!$1" He hardly has to act in these roles. But give him an odd character like the guy he played in Tropic Thunder, can't remember the name, and he knocks it out of the park.
Many actors are seen as great based entirely on the parts they play. For example, Daniel Day-Lewis is one of our most respected living actors for the parts he played in films like "The Crucible," "Gangs of New York," "There Will Be Blood," and most recently "Lincoln." He certainly deserves the reputation as his performances are almost always stellar, but I wonder what would happen if he ever took a smaller part in a more poorly-written movie. You see, Day-Lewis has made his career so impressive by carefully picking the best, most challenging parts and doing a movie every 2 or 3 years. He keeps himself sterilized and safe behind good screenwriting.
Samuel L. Jackson rarely gets called a "good" actor. A lot of people like him, and he has a reputation as a badass dude, but how many people even know he was nominated for a Best Actor Oscar for "Pulp Fiction?" He is a damn good actor with much more range than he gets credit for, but his more infamous roles such as "Snakes on a Plane" and "Afro Samurai" are where he gets the reputation of a less serious and therefore less talented actor.
I think a lot of supposedly "bad" actors and actresses have the potential to really surprise you, they just need to be given the right part. Now, obviously some actors are still better than others. Keanu is pretty terrible in almost everything; he worked well in The Matrix because he is a man who has lived an empty life and is being shown all that he has never known. His lack of emotion is appropriate as his role is more of a symbol than a man. But when he was called upon to have more emotional range in the sequels he sort of fell flat.
So, I kinda think that it's impossible to say who is the worst mainstream actor. If they are mainstream then they were at least good enough to convince a casting director.
On the other hand, Taylor Lautner is a useless sack of meat, and I'm not sure Sam Worthington knows what acting actually is...
