As the title says. Who irked you the entire series? Which character got under your skin or felt useless or just gave you the perception that they made the show worse by being present?
Then, how do you feel you'd make the character better? Not simply removing them from the show but making them more...
Dawn.
1. Make her Buffy's sister at the start of the show or have her related to one of the other characters
2. Just not have her
I can't pick between Dawn, Riley or Faith.
Dawn was a drag and I never really got used to her abrupt supernatural insertion into all their lives. Like the reply above me, either have her be the sister from the start, or not at all.
Riley: THE most boring boyfriend in the history of fictional boyfriends! So perfect, so good, so "sir yes sir." I groaned out loud when he shows back up again later on. I don't think there's a fix for a character who is supposed to be the perfect, normal, golden-boy potential "hubby" for Buffy.
While Buffy could be too goody-two-shoes at times, I hated Faith for being so 180 degrees the nasty shit-stirrer, and all the jumping the bones of the guys who matter to the other characters. B word all the way! I guess they had to have someone who was the dark to Buffy's light, so I have no suggestions for how Faith could be different; I just couldn't stand her lack of ethics.