The worst "name" actor in an otherwise good film or show would have to be January Jones as Betty Draper in Mad Men. Fabulous show, but she's darn near unwatchable, even if she is cute. It's jarring, frankly, to juxtapose her wooden line delivery with the splendid performances by the other actors on that show. Andie MacDowell is pretty damn bad, too.
Among men, Keanu Reeves is pretty much the standard, though he was perfect for the Bill & Ted movies, and even the first Matrix movie, because those characters were SUPPOSED to have a bewildered, blank look on their faces.
Oh, and Megan Fox is legitimately hot. I've seen her in person. Even if she weren't famous, if she walked into a bar every guy would stare at her -- including you, NP666. (Cameron Diaz is another story -- quite fugly. If you saw her in a bar and didn't know who she was, you'd have be drunk to even think twice about her.)