It's hard to pinpoint a worst actor, but I can tell you who the worst actors are, in proportion to how much they're paid. Tom Cruise is a bad actor who makes a mint. In this case, bad actor means "unconvincing". Like a cheap special effect, unconvincing acting makes you say "that looks totally fake", removing you from the movie. In that regard, Tom Cruise is to acting what a saucer on a string is to special effects.
Second place goes to Keira Knightley. For the longest time, I assumed Keira would have been okay if it were not for the fact that she is perpetually miscast in roles she clearly can't perform - bounty hunters and pirate queens. I thought she'd only be capable of the "posh totty" kind of role. But then I saw Pride and Prejudice and The Governess; roles she should be perfect for. She was still terrible. She fails for the same reasons as Tom Cruise; so unconvincing, it destroys whatever credibility the movie had.
I give other bad actors a pass, either because they're poorly paid amateurs, or because the movie they're in is too bad for them to ruin. Also, I forgive actors who can only play themselves, like John Wayne or Vin Deisel - they often can play themselves very well, and that works just as long as they're put in a movie that happens to need John Wayne or Vin Deisel. Hell, that even applies to the modern shitty stars, like Shia LeBoeff or Michael Cera. It's only when actors demand huge paychecks in high standard productions, that I expect a high standard of performance.