Seems to me that Gin would win this overall; he's all around very strong and from what we've seen his Bankai may just be the strongest we've seen so far. While Hitsuguya and Mayuri both have AoE techniques at their disposal Mayuri's poison cloud's range is fairly limited and a lot of Hitsuguya's moves are either predictable and easy to dodge or take a while to use (though I agree that the TJ move is quite overpowered). In comparison his Bankai has both speed and long range (even if it isn't as long as or as fast as he told people it was) and is just all around a better fighter than both of them.
So until we see either Shunsui's or Ukitake's Bankai (which knowing Kubo will be ridiculously cheap) Gin would probably win, though there's always an element of luck in a free for all so who knows .
Shunsui doesn't need bankai to win. He'll hide in the shadows, wait his turn and stab anyone. If you paid attention to the manga, a shadow can be cast from the user, and from any distance, Shunsui can appear, as long as he steps on the shadow. Then if he plays his color game, no one can attack him unless they want to be attacked back. And within the free for all, who is really going to recognize that you have to call a color to attack him. They'll be cutting nothing.
er.... Mayuri just poisons them all ?
It deffinetly goes in Shunsui's favour. He is the second strongest Captain in terms of fighting strength after Yama now according Databooks and is proven by the manga.
I also agree with freshseth83 on the scenario he suggested. It's the method Shunsui will use in this situation and he can easily take the majority of the Captains with this and then leave for the last the weakest as Mayuri and Komamura. He can take both pretty easy.
I can see Gin, Shunsui and Soifon winning.
1) Gin - point and shoot.
2) Shunsui - being sneaky in the shadows and overall superiority.
3) Soifon - being sneaky and using shikai to take people out while they fight others or going far away and shooting her bankai into the melee of captains.
Byakuya, Gin, Mayuri and all have the highest chance of victory.
Byakuya has countless blades at his disposal that could very well flood the battle field.
Gin has the highest chance of victory in this group. He could just swing his blade horizontally in a 360degree angle and overwhelm most here.
Mayuri has his bankai and it's poison, even if some one destroys it, he still could have traps laid out prepared for almost any situation.
Shunsui the most experience fighter here could simply hide in his shadows all day (as cowardly as that sound), and sneak attack anyone in the vicinity with a Kageoni stab or Irooni slash.