I'd say Ezri Dax, but as she's a Dax it means she's bound to change. Anyone who read The Lives of Dax knows this.
So I'll have to settle for T'Pol. From being a royal pain to actually establishing a family with a human to vulcan transspecies convert.
I'd have to go with Ezri myself. She basically got completely reinvented in the novels. I'm still not completely sold on her just suddenly "clicking" and becoming one with symbiont and being nothing like the character we saw in season 7, though from a dramatic standpoint I can understand how constraining it would be to have a main character be a less than effectual counselor with multiple issues of her own, the main of which was living in Jadzia's shadow.