Kira and O'Brien for me. Couldn't stand either of them throughout the entire series. Same with Vedek Bariel and Winn.
I also didn't like Weoyun at first but he great into probably my favorite character on the show if not in all of Trek.
Worf did get a little annoying as someone else stated, but he's not as bad as the other two for me.
I've always liked Bashir, Garak, Odo, Quark, Jadzia Dax, and Sisko throughout the entire series. Oh and for bad guys, Gul Dukat was great.