The Big Bang Theory - Sheldon Cooper
I'm pretty sure that The Big Bang Theory was originally envisioned more as the Leonard and Penny show with Jim Parsons' Sheldon meant only to be present for the occasional extra laugh. However, for me, TBBT has turned into The Sheldon Show. I find most of Leonard's and Penny's storylines to be somewhat dull...unless, of course, they involve Sheldon. Sheldon has all of the show's best plotlines, the funniest lines and with Parsons' pitch-perfect delivery, has turned the guy from what could have been an obnoxious sidekick into the show's best and most endearing character.
Weeds - Doug Wilson
Weeds has wavered in quality these last few seasons, but the one thing that has remained constant is the show's supporting cast. Since I have to choose one I am going with Kevin Nealon's Doug Wilson (though Justin Kirk's Andy Botwin is a close second). While the show for me has taken a tone lately that is no longer all that comedic, these guys have kept the funny in Weeds. Nealon especially is a complete laugh riot and no matter how much Weeds falters, I would watch as long as he remains. Just read some of Doug's best quotes and you'll begin to understand his brilliance.
How I Met Your Mother - Barney Stinson
Much like TBBT was probably meant to be all about Sheldon and Penny; I think that HIMYM was originally meant to be more about Ted and Robyn. But...it has been Neil Patrick Harris's Barney that has completely stolen this show. NPH's womanizing Barney has turned out to be a comedic genius...and in these more recent seasons especially, he has added some depth to Barney's character that makes him all the more interesting. Who ever thought that we'd all be rooting for the Lothario?!
Brothers & Sisters - Kevin and Scotty
Some of those brothers (*cough* Tommy) and sisters (*cough* Kitty) are more grating than others, but for me, my absolute favorite character on B&S is Kevin (and I am cheating by adding Scotty here as well). I find Kevin's story lines to be the most interesting, honest and entertaining on the entire show. Furthermore, I find the Kevin and Scotty relationship to be one of the best relationships on TV - gay or straight. Who wasn't bawling during last season's wedding episode?
Mad Men - Betty Draper, Peggy Olson and Joan Holloway
Mad Men may be a show seemingly all about men from its title, but it's this trio of strong women (so I'm really cheating here) that often steal the show for me. I honestly couldn't choose between them. Earlier in the first season, I had written off January Jones's Betty as one of the show's weaker characters, but as that season (and the second) progressed, her tortured, sometimes manipulative Betty has evolved into one of the show's most fascinating characters. Same goes for Peggy and especially Joan Holloway. In the world of Mad Men, these girls can hold their own; they also prove that the men aren't the only one with the problems, nor are men the only ones that lead interesting, complex lives.