Paris Hilton.
I came in to mention Alex Krycek. It seems like he and Mulder always got into knock-down fights whenever they saw each other, plus Krycek got beat up by Skinner (and handcuffed to the balcony rail), then had his arm cut off…
Sydney Bristow from Alias. Tortured so many times it was ridiculous.
Wasn’t there one though where Charlie Brown was invisible and he finally got to kick the ball? Though maybe that was the comic strip.
Ben from Lost might be a good one, if for nothing else then getting beaten on by everyone else on the show.
Both good nominees, but there’s a difference: it really seemed to HURT when Jim Rockford took a beating. Rockford was tough, but he wasn’t THE toughest guy around, and he knew it. There were guys he KNEW he couldn’t beat, and he often got the worst of it in a brawl.
Mannix, though, was a more cartoonish macho guy. You KNEW he was always going to come out on top in the end. He might get beaten up or shot, but in the last scene, you just saw his arm in a sling, and you KNEW it was “just a flesh wound” that he’d recover from in no time. You ALSO knew that, however bad a beating Mannix took, the other guy looked worse.
Rockford was much more human, which is just one reason his show was so much better.
It was one of the cartoon specials. (Of course, Peanuts fans don’t count this because we all know only the comic strip is canon.) 
Adam West’s Batman. He always had at least two fights with the baddies, sometimes more, and lost all but the last one.
Giles always got hit in the head, although it’s probably because he never locks his door.
This. They are approaching Evil Dead levels of fake blood and gore on Supernatural.
This may be too old for most to remember so unless reruns and syndication have brought it forward you’ll just have to take my word for it.
Cheyenne Bodie, played by Clint Walker, was a huge dude who managed in every episode to get beat up at least once, and often by a smaller less imposing guy.
I can’t be quite as clear on the Mission:Impossible guy Peter Lupus (we called him Crate Pusher) but another big guy who was forever being accosted by little folks.
10-4! or Britney! 
Rupert Giles.
Giles was the one I came in to suggest. For non-main characters, Cole Barker was only in Chuck for two episodes, and managed to get shot twice, tortured twice, hit by Sarah and knocked out by Casey.
The rest of the time, it was the “Torture the Audience Show”.
No mention yet of Homer Simpson?
Ben Linus was definitely the guy I thought of. BTW, in the scene where he got beat up last ep, Cusick accidentally actually popped him, and he’s got a huge shiner.
–Cliffy
Scratchy. Probably survives for less then 30 secons on average per-appearance.
Itchy’s a jerk.
Bruce Dern.
Actually, I don’t know that his characters were always getting beaten up; maybe I just wish it were so.
I came in for Giles as well. As many times as he’s been knocked out, it’s a freakin’ miracle he doesn’t have massive TBI.