>>14460 (OP)
Kirk
On a level playing field he's probably the most formidable. He's physically and mentally fit, has dealt with shit that would turn the other four pale white (even Sisko). He also has a superb crew, probably overall the best crew. One weakness is the Connie, which while being the best looking starship of the five on hand, it would be horribly outclassed by everything except Archer's Enterprise.
Picard
Picard's too weak for a death match unless the opponent has wires in their face. He's also not in the best shape, healthy but more towards the arugula-eater end of things. On ground ops he's not in a good position. However, he does control a Galaxy class which can handle damn near anything you throw at it. Additionally, while his crew is generally less impressive than Kirk's, he does have Data and LaForge as a strong double act.
Sisko
Sisko could pretty much rip everyone apart on the ground, save Kirk, easily. His ship is also the only purpose-built attack ship, making a major concern for Picard's Ent-D, and space combat equivalent of a sand blaster for everyone else. He's also, even unlike Kirk, willing to use Romulan-tier-ethics in his operations to win (giving him additional battle options). His main weakness is his crew, while quite competent, may not be able to compete against the guile of Kirk, or the capabilities of Data.
Janeway
She's not the best commanding officer, being prone to unwise, rash decisions. Her ship is more advanced than the Connie and NX, but isn't nearly combat-oriented as a Galaxy or Defiant. Her crew is competent, but also unremarkable as well. On the ground she is just about able to hold her own against non-Starfleet types but against the other four she's not a great pick. Really her greatest strengths are her EMH and her Borg crewmember(s); they could provide a devious wildcard that would throw the match their way.
Archer
He's sort of midrange on the physical fitness end of things; better than Picard and Janeway, worse than Kirk and Sisko. He's got a comparatively crap ship that all the others can out-stroll (let alone outrun) and out-fight. His crew is fine, but doesn't have any bright spots compared to the other four. What he does have though are time traveling friends, and the fact he might be a time traveler too who later develops a Sisko-tier morality plus cynicism at the same time. Also the less-sensitive era the crew hails from gives them access to the MACOs who are a little more red meat than your standard security detail later on.
Results
I think in most instances, Kirk probably wins or fights to a draw. Future Guy Archer can't erase Kirk and with that edge gone, NCC-1701 and Kirk's pecs would ruin the ENT team. Janeway would be similarly dispatched, probably after some toddler tier trickery since a Connie isn't going do to much to an Intrepid in a standup fight. Data on Ent-D would be a major problem, as would Sisko in general, however in both instances Kirk's power of ass-pull would probably win out. At least it would be a fight to a draw.
Archer can probably take Janeway with some effort, maybe even Picard if he can take it to the ground, but Archer has no hope against Sisko. Picard and Sisko could fight to a draw with outlier win possibilities for both of them; J-L getting an obvious win on Janeway. Janeway herself might be able to use her combination of Borg, photonic and maybe even time travel shenanigan technology to cause some unexpected problems for Sisko…as long as the battle is kept on the air. If it goes to ground, Janeway is doomed.