I'd still give Stewart the edge in terms of performance (his strong and authoritative presence is what really made that show work, and gave it the weight that it had), but yeah in terms of characters I'd agree Sisko's was the most complex and interesting.
Patrick Stewart, as an actor, equals "great." Captain Picard, on the other hand, was often a victim of uneven writing that sometimes undercut the quality of Stewart's performance. I would be the first to say however that Patrick Stewart elevated TNG. Of course that's just my opinion.
Which makes it kind of amazing then that he basically got pushed into the background for awhile after the pilot, because the writers didn't know what the heck to do with him. Lol
Well, to be fair, a common weakness of all the modern-era Treks early on has been the sort of shotgun appraoch to its writing wherein they tried to focus on everyone and see who audiences gravitated towards. This was especially true of the first half of the first season.
My theory for this was because modern Trek didn't seem to go through the normal developement process of write the pilot, re-evaluate, make the pilot, test it, re-evalute, then go into full production. They always seemed to go straight into full production with the plan of re-evaluating and ironing things out on-the-fly. On the plus side, it gives fan reaction more weight. Bad side, the writing tends to be generally softer. Again just a theory of mine.