Two of the worst notable Hollywood actors today are Kristen Stewart and Taylor Lautner, coincidentally both main characters in the Twilight films. Aside from being mainly famous for starring in that car crash of a narrative, they're just so bland and can't even emote.
Arnold Schwarzenegger. The reason why he's so entertaining and funny in his films and just how quotable he is isn't because he's by any means a good actor.
Michael Cera. I don't know whether or not he's particularly a bad actor per se, but I just find him annoying, can't understand why he's so popular, and he's part of the reason why I didn't like Scott Pilgrim vs. the World.