Thinking of 5 was pretty hard. I'm the type of person where I tend to like everyone in my fandoms. If I disliked a character a lot, I simply stop watching the show (see: Meredith, Grey's Anatomy and Marisa, The O.C.).
1. Rose Tyler, Doctor Who. Perhaps I was expecting too much considering, at that time, all I read was how awesome and wonderful Rose was and Rose+Doctor=OTP4EVA!!!!! I also started in the DW fandom with Series 3 and I LOVED Martha but the constant comparison to Rose (in canon & fanon) and how no one could be as brilliant as Rose... UGH UGH UGH. But really, Rose grated on my nerves. I hated how she treated Mickey. I hated how cliquey the Doctor & her got, like they knew they were awesome and let us make you feel like an outsider with our cute little inside jokes and whatnot. And then fact that RTD just wouldn't let her go! I also don't get the appeal of Billie Piper. At all.
2. Castiel, Supernatural. This is more of a 'too much of a good thing'. I liked Castiel at first. But then he kept coming back, more and more frequently until he was made a series regular! But I think what got to me the most was the fandom. For a fandom that constantly harps on SPN for being misogynistic, it sure loves to hate 99.9% of the female characters -- "I don't want no stupid female to come between Dean & Sam!! This show is about brothers!" -- but introduce a good-looking male portrayed by an actor who enjoys playing up to the fandom, the chorus of "OMG Dean/Castiel is soooo hawt! Sam who? Get rid of Sam, we want our D/C!!!!" really annoyed me.
3. Uther Pendragon, Merlin. It said "your favourite fandoms" and Merlin is one of my faves. So, if I had choose someone, it would be Uther. His "all magic is evil and must be destroyed" stance is getting tired and old. There hasn't been a lot of growth/progression with this character over the last 2 seasons. Here's hoping for S3.
4. Rebecca, Oli/Christian fandom. This character is just plain annoying. Her insipid crush on Christian was stupid and lame. Most of the blame goes to the writers -- if one character falls in love with another character who is already in a relationship, then one would expect a storyline involving a love triangle or evil!third party trying to break up established couple or something. This is a soap opera after all. But none of those things happened with Rebecca. She just mooned over Christian for months on end. Booooooooring.
5. Kate, White Collar. We never found out much about Kate so I never understood why Neal would risk so much her. Why were they even in relationship? And then the show blew her up. Buh-bye.
Something different from the preceding fandom rant-age. Have some happy, my fellow DOWs! Video of Zac's Details photoshoot: