1. We all thought that shunsui's shikai has to do with wind and it turned out completelly different. My bet is that there is more to uki's shikai. Furthermore, even if the shikai does not work, it does not matter. Uki has shown to be skilled with kido and his sword skills are definitely not lacking either.
2. If we are going to do a hypothetical match-up, then there should be nothing left one chance. That means, no sickness affecting uki.
3. Ken is strong, but he can't hurt what he can't hit. Furthermore, if you think he can tank hits from a senior captain, that is wrong
I will note that Kenpachi is RIDICULOUSLY slow. Probably the only slower captains are Mayuri and Komamura. I don't even think Kenpachi knows Shunpo. All of the opponents he has fought have been big, and slow, or stupid. Or all three.
Basically, although Kenpachi has looked pretty great in all of his fights, they have all also been wildly in his favor. Strength based match ups with a guy who is, let's face it, the most brute-force based character in the series.
I will say Zaraki isn't the fastest guy, but he isn't 'slow'. He can keep up when needed to. I would say his awareness is pretty high. I bet he can sense reaitsu and would be a match for Ukitake, but Ukitake knows kidou, and is probably as efficient as anyone in it. He's calm mannered which is the opposite of Zaraki also. I think they are almost opposites. The only one I would say which is more opposite than him is Byakuya. I think it'd be a drawn out fight that Ukitake wouldn't find a reason to kill Zaraki. And Zaraki would get bored and say something like, See ya later, I aint gettin nothing outta this fight.
Going by what we've seen so far - Kenpachi.
But I still think Ukitake would win once Kubo decides to focus on his properly and make him fight at full strength with Bankai etc
Ukitake use probably most kido spells without incantation. He knows, that fighting in close combat vs Kenpachi is very risky, and that Kenpachi doesnt know any kido, or long range attacks. Ukitake is faster then Kenpachi and has more battle experience. Im very big fan of Kenpachi but I personally dont se him winning this. Ukitake just has to stay away from him, and spam kido spells. Kenpachi is doomed.