Well, my heart balance between Kira and Jadzia.
- Kira already held the position of Second in command (under Sisko) on DS9 base so,it would be normal that she takes the position of XO on a Starfleet vessel... provided that she is more temperate in her positions (besides, she seemed more moderate, measured, at the end of the series).
-> a First Officer isn't there to put obstacles in the way of his/her commander under the pretext to share different opinions/ principles/strategies, right?! His/her role rather consists to facilitate the management of the crew and its environment, to help the Captain to focus on his/her mission.
- Jadzia Dax because 1) she had the rank (the 3 pips of Lt Com.) to hold this position 2) due to her very large knowledge/experience , she could easily help/advise, regardless the position 3) she was someone calm and composed so, she could be a great support for the Cpt.
And if you allow me, I'd add the name of Tuvok, who was the perfect candidate to this positon based on 1) his seniority in Starfleet 2) his knowlegde of the crew and his Captain (it is in the best interests for everyone to rely on a good psychologist who is able to put himself/herself in the position of others to get better results).
Seven, as brilliant and capable as she is, knows nothing about military discipline version Starfleet. Plus, I have noted that her large knowlegde was mainly technical (to intervene on equipment, programs and developing startegies had no secret for her but on the field, she was very seldom decision maker and I think it suited her well -> she prefered and used to use her brain more than her arms and weapons).
