Close tie between Janeway & Picard. If I had to choose one, it'd be Janeway. I know trekkies tend to rank Janeway/Voyager in general as their least fave, but idgaf. Janeway is equal parts compassionate and doesn't-take-shit from anyone, and you could say they all are, but she does it best, imo. In terms of sheer endurance, all the other captains can suck it. (:
Why do you like Sisko, out of curiosity?
There's something about his speech patterns that bother me. It has sort of a dream-like quality most of the time; even when he's mad, I can't really take him seriously as being mad. He'd make a great philosopher/orator; a captain, not as much. Just my opinion though. Other than that, nothing really stands out either way.
I said Sisko hoping to get a reaction, I never really watched DS9.
I liked Janeway for much the same reasons you mentioned, but the thing about her that enthralls me the most about her is like you said - her erratic nature. There were some very good concepts that were worked into those episodes, especially in one particular episode in season 2 called "The Thaw" where she has to outwit and defeat the embodiment of fear whose literally holding people hostage and could kill them by literally scaring them to death. I think the setting and kind of universes inspired from Gene Roddenberry is lost in the new JJ Abrams films, which is a shame.