Sisko did some pretty bad ass things (as mentioned- poisoning a planet, punching Q, facing down a Dominion fleet with one ship etc) but I wouldn't feel "uncool" if we were to hang out, ya know what I mean? He was bold. Sure. There's no questioning that. But was he really bad ass? When I hear that term I automatically think of Jim Kirk.
He was a womanizing, space faring cowboy who I would feel totally lame hanging out with. (Note: I'm not talking about William Shatner. I'd feel okay about hanging out with him.)
My vote goes to Kirk with Sisko in second. The other Captains, while each great in certain respects, do not meet my standard for bad-assery. I can conceede that they each had their bad ass moments though. (Janeway with her guns, Picard deciding to ram the Enterprise into the Scimitar, Archer cutting off Phloxs' head...)