Ask me in 1993 I would have instantly said "Oh, definitely Ro". Ask me now I can't imagine the show with Ro instead of Kira. The entire show would have taken on a different character had one of the main characters not grown up as a terrorist. There are three characters who you couldn't have taken out of the show without completely changing its feel, and Kira is one of them.
It would have been great to see Ro in a recurring role though. She joined the Maquis from an emotional outburst, she could have had an interesting storyline having regrets about her decision, but still realizing she could never really accept the restrictions of being part of starfleet.