Extremely tough to pick a winner. Both of them definitely possess advanced levels of Busoshoku, and Mihawk has been portrayed as having high level Kenbunshoku. Oden has Haoshoku but without its advanced version it won't be of much help against Mihawk (which I'd be surprised if Mihawk didn't have Haoshoku).
Oden might be slightly stronger but I'd say Mihawk is skilled enough to win with extreme difficulty. Mihawk wins 6/10.
I wouldn't be surprised if he had that as well, but his Kenbunshoku seemed to be at a level not seen by other characters. In the midst of that chaos he narrowed his vision solely on Luffy and even made sure his attack only hit Luffy. It seemed impressive to me.
Kaido considered Oden to be on the same level as Yonko+ level fighters. Oden couldn‘t easily beat Kaido considering it took an extended battle to beat him. They were also shown clashing evenly when Oden was enraged upon returning to Wano, and Oden decided to embarrass himself as opposed to striking down Kaido then and there. This can only make sense if Kaido was already around that level of power.
They're probably on the same level. Kaido probably powered up a bit in the 20 years since killing Oden but it's not made out to be a significant amount. He was already approaching his prime.
Well, it's still exceptionally rare when you consider that only the top tiers have it amongst the entire population of the world. According to the wiki there are only 12 confirmed users alive.