We know nothing about Kaido's powers, except that he's called the strongest ... in the world, and is hard to kill. He also probably has a legendary Zoan of some kind., but thats' just guessing. We also don't know how strong BB is after the timeskip.
I'm going to go with Kaido, but i do thing that BB could hurt him decently. In his big appearence, he's grumbling about Whitebeard doing a number on him, which either means he can be damaged by the Gura Gura No Mi, or Edward Weevil is a certified badass.