I think it's a toss up between Picard and Sisko.
Picard has already a tactical maneuver named after him and Sisko was an accomplished military mind who got (deservedly) much credit for ending the Dominion War.
Picard is a diplomat first but he is experienced and sly enough to know not to show his entire hand and have backup plans in case he needs them. He's quick on his mind and able to improvise should he have to.. it will be very hard to beat him in any case.
Sisko is good at the strategic and tactical level.. able to lead an entire fleet as well as a single ship. He is not afraid to make the hard choices when he has to and he's able to see and call a bluff.
Kirk is to emotional for me to be a really good tactician.. he's far to prone to use blunt force to win and sometimes that's not the best way to go. He's a bit like Patton.. capable and a legend but gotten there over too many bodies.
Janeway i don't know.. she surely can stand on her own and she's highly intelligent. To be honest i'd rate her middle class but being Starfleet that middle class may be top tier for many other organizations. Since i think Voyager was badly written many times, i.e. too ridiculous odds against the Voyager but she still wins the day it is hard to truly judge her.
As has been said Riker is overlooked.. he's easily one of the best officers Starfleet has and only recently got his own ship. He's very smart, quick wits and he rarely plays by the book and likes to improvise (to great success). That makes him a brilliant tactician and given a bit more time will easily fill the shoes of Picard and maybe make his own impression on Starfleet.