That's hard- there were so many. I wouldn't even try to recall the guests or background actors who sucked.
Eric Bana as Nero. The name was stupid. His odd drooping mouth was stupid. His monotone voice throughout the whole movie. Fuck, he was aweful.
Shatner was aweful at times, surprisingly awesome other times.
Oh, I could look it up, but I'm recovering from vertigo so I won't. But Captain Klang or whatever he was called in Star Trek TFF. He was pretty bad.
OH WAIT! The super hot Romulan babe at the beginning of TFF... She was the worst. I still cringe everytime I hear her voice in that movie. Smokin' hot though. Hottest Romulan ever.