I actually preferred Ezri to Jadzia. She brought something new and fresh to the cast dynamics during that final season and Nicole De Boer played her part very well (she could easily have taken the Buffy/Willow smartass teenager path).
Most of my hatred on the show was reserved for Kai Winn but, as said before, as a viewer you were supposed to hate her. She was morally repulsive for a reason.
For me though, the blandest, dullest, recurring character on DS9 has to be.... Vedek Bareil. Unconvincing as a holy man and as a lover. What the hell did Kira see in him??! Even the Mirror Bareil was boring! I never re-watch episodes featuring him and was glad when he died (which was also the longest and most boring death scene ever seen in Star Trek!).
And..... breathe. :-)