DISCLAIMER: This is not an exhaustive list, nor are these in any particular order. (Basically these are just the five that came to my head most prominently among the TV shows I am most familiar with.) Please feel free to add on in the comments!
+ Sue Sylvester [Glee]
By now we all agree that Glee is the worst thing ever and probably had a short life expectancy from the get-go. The only thing holding this string of unrelated high school drama vignettes together was the music and Sue Sylvester. Her "I hate everyone" attitude made Glee relatively bearable, as we, too, hated everyone at Some-Random-President High School. As I grew to hate Mr. Schue even more with each passing episode, Sue's attacks on his abomination of a haircut became more and more satisfying. She said what everyone was thinking, and I appreciated that. (I use the past tense since I stopped watching Glee two seasons ago due to the fact that the music was no longer compensating for its horribleness.)
+ Nanny Carrie [One Tree Hill]
Having maxed out the limits of believable drama in previous seasons, One Tree Hill really had to go big or go home if they wanted to keep the momentum going. So where did they go next? Psychotic nanny. After skipping four years of time so that they didn't have to figure out the whole college thing, One Tree Hill added a new character to the cast: Nathan and Haley's son, Jamie.
And you can't add a new character to OTH without giving them some sort of major life trauma. Being kidnapped twice by a manic, delusional nanny is the perfect solution. The first time, resident bad-guy Dan Scott, Nathan's dad, saved Jamie, so of course Nanny Carrie had to RETURN (bum bum bum) and try to kill him. OTH really outdid themselves here, and to this day it's the most outlandish plot line I can remember from the series. And trust me, they had a lot.
+The Xerox Girl [Friends]
Whether you agree with Ross or Rachel on the whole "we were on a break" thing, let's just all come together on the fact that it's all The Xerox Girl's fault (also Ross's fault, but let's focus on The Xerox Girl for a minute). If it weren't for her, maybe Ross and Rachel would have figured themselves out a lot sooner and not been the trainwreck of a relationship they were.
The Xerox Girl may not have known that Ross had just fought with and/or broken up with Rachel, but she was aware that she (Xerox Girl) and Ross were not married, in love, or even dating for that matter. And Xerox Girls shouldn't just go around sleeping with depressed guys they don't know. She had a say in this, too. So thanks a lot, Xerox Girl, for ruining the most iconic relationship in sitcom history, no matter how nauseating it was.
+ Devon Banks [30 Rock]
Not only is Devon Banks played by Will Arnett, who can do no wrong (as evidenced by the fact that he scored Amy Poehler), but he is the perfect anti-Jack. Both Jack and Devon are strong executive types, but Jack's extreme conservatism is counteracted by Devon's work for the Obama administration and his "gaybies." Their childlike arguing and debate over trivial things makes their pairing endlessly entertaining. A favorite of mine:
Devon: "Revenge is a dish best served cold, like sashimi or pizza." Jack: "You prefer cold pizza?" Devon: "The morning after? It's the best." Jack: "Better than hot pizza? That's insane." Devon: "You don't tell me what kind of pizza to like!"
+ Chang [Community]
Benjamin Chang. What he lacks in skill, he makes up for in pure, unadulterated madness. Like all great villains, his antagonism stems from deep personal trauma and hurt, leaving him thirsty for revenge. [In this case, being jobless, demoted to community college student rather than teacher, and then being denied acceptance into The Study Group no matter how badly he wanted it.] It could be argued he is simply misunderstood and wants to be accepted, but his unstable personality and impulsive actions make him impossible to accept.
He hit rock bottom, living in Greendale's air ducts and janitor's closets with Annie's Boobs (the monkey, guys. It's the monkey's name). Commence plot for revenge. He soon devolves into a string of psychotic rampages that escalate quickly time after time, until he loses all sense of morality, captures and replaces the Dean with a decoy and runs Greendale with an army of militant 12-year-olds. His sheer unpredictability and madness, not unlike Ledger's interpretation of The Joker, make him one of the best TV villains across the networks today.
Who do you think is the best TV villain?