The characters/actors who can fight believably and well, vs those that can't.

One good fighter is Cameron from Terminator: The Sarah Connor Chronicles. Now, she is a Terminator, so she will be good automatically, but most of her fight scenes are good, and Summer Glau is pretty convincing.

An awful one is Lana Lang from Smallville. Now, Kristen Kreuk tries her best, but I will never buy Lana as this kung fu powerhouse, no matter what the show says.

I don't know how long ago I first saw this in the Supernatural thread, but I find myself saying nearly every week, "Sam's getting pummelled. Must be Thursday."

I liked Alias a lot, but I've never believed Jennifer Garner as an ass-kicking ninja. She just didn't have the physical nature for it. But I think she's great in romantic comedies.

Dani Reese on Life, on the other hand -- I buy her as a tough cop who can handle herself just fine.

Worst: David Carradine on Kung Fu. Slowest moving martial artist ever. Hard to believe they picked him over Bruce Lee.

Wow. I totally forgot about Starbuck (BSG). Going up against a Cylon and she struggles. However, she was pretty badass during the mutiny.

Ya'll might want to check this out: : http://news.bbc.co.uk/2/hi/entertainment/7954788.stm
It's an interview with the fight coordinator for Robin Hood about how they choreograph and rehearse these things, with lots of video of one particular fight scene in rehearsal. There are lots of good moves to be seen.