Kira. Kira was believable as a tougher fighter than anyone in a Starfleet uniform (sorry, Worf). Also compassionate and spiritual and patriotic.
Jadzia was the sort of pretty Hollywood likes. As others have said, she was portrayed as perfect at so many things it became unbelievable. Real characters have strengths and weaknesses. To me, she was only marginally believable as a science officer or senior officer and had a lot of trouble as a 300 year old wise person. A hard role for any actor, and for any writer. While she had some good scenes and some good episodes, but overall not the strongest character. She had trouble seeming emotionally invested in what was happening, like in Sword of Kahless she didn't seem to care much whether any of them got out of the cave or what happened to the sword, the only surprise was that she went on the trip at all.
In Facets, the interesting parts went to all the other actors, especially Auberjonois; that episode should have been great for Jadzia but instead she just seemed like filler.
Ezri at least had some strengths and weaknesses, and I think showed off her joined trill nature more successfully than Jadzia. Maybe it was a writing team that had learned better, but I think a better actress had a lot to do with it.