I like O'Brien because he's such a great everyman and played by such a brilliant actor.
Bashir has to get a mention as being one of the best developed characters in all of Trek. He convincingly developed from being an obnoxious know-it-all rookie to a compassionate and humane man.
Sisko for just developing into a great captain and generally being a bad-ass.
Ezri must get a mention for making such an impact in just one season.
Picking a 5th one is very hard. But rather than one of the regulars, I think I have to opt for Garak. Surely this choice speaks for itself (and tells lies, as it does so)?