Ichimaru Gin vs. Kaname Tousen: Gin's Bankai is supposed to be that he shoots Shinsou into the air and a bunch of other ones come raining down, right? Or at least, that's what I've heard. (I think it was a move in one of the games. It's probably not official though.) It seems to me that, like 11th Division is based on fighting and 4th is based on healing, 3rd Division is based on speed, thus, Ichimaru's zanpakutou is based on a projectile, and light sword which was meant to grant stealth and speed. Kira seems to have a zanpakutou that's meant to slow others down, which technically speeds him up.
I would say Gin would win that fight, just because I'm a fangirl. :3
But in reality, I'd imagine Tousen would win, because Ichi-chan is kind of cowardly and would probably freak out if he lost his senses.
Yoruichi vs. Aizen: Aizen man. He's got a billion arrancar to sit on, and if it's a one-on-one, he's still like, the super gary-stu-all-powerful-super-smart captain, and Yoruichi is... well... 100 years out of practice. DD: That's not to say Yoruichi isn't kick butt, but Aizen is just more kick butt. ^^;
[/Whooo~ I posted.]
Yachiru vs. Izuru Kira: I'd have to say, Yachiru, because Kira seems like just such a timid shivery person that he'd be too ashamed to hit Yachiru. xD; Even though Kira is one of my favorite character's, he isn't exactly the winningest character in battles, especially not against Yachiru, who's like, super guilt-free.
Ulquiorra vs. Aizen: ULQUIORRA FTW *FOAMFOAMFOAM* But honestly. It'd be a long, drawn out, suffering battle, but Ulquiorra is probably the loser, considering he's already under Aizen's control. But it's definetely a battle I'd like to see~ Ulquiorra would have to whip out some 00ber attacks. :3