I don't see any way anyone could possibly have been worse than Terry Farrell, therefore, I pick Famke Janssen.
However, I don't think Janssen is a particularly good actress either...

It's not like Dax added anything to the show or was relevant to the plot in any way whatsoever anyways.
...the Dominion War, the Emissary, the general operation of the station...
One of Starfleets senior officers on DS9 (before Worf, perhaps most senior after Sisko?) and a capable pilot and commander of the Defiant; an old mentor of the Emissary who he often turned to for guidance; again, a senior officer on the station and with centuries of experience.
Re:OP, nah, I quite liked Farrell. Janssen is great, but Farrell did a good job with the Errol Flynn-esque aspects of Dax where Jannsen is much more serious. Though perhaps as a Trill guest-star....
I wonder, have the Kriosians ever been re-visited?