I'll guess Rayleigh. Not going off much, apart from Rayleigh being on par with Kizaru. Idk how Rayleigh would fight against the Darkness fruit or if the Darkness fruit is even going to present problems to him particularly. He doesn't have a DF anyway, so he won't lose any powers and I'd think his haki would be strong enough to let him deal with Blackbeard's Darkness-based offense. He's also too fast for Blackbeard and although Blackbeard is physically strong and durable, Rayleigh I would imagine has the ability to attack with the kind of power only seen from the likes of Admirals and Mihawk, although can't say for sure (he could just be fast and have strong haki, but not be that strong). The Earthquake fruit I just think Rayleigh's haki would be enough of a defense. Again, don't really know because we've hardly seen what Rayleigh can do and same for Blackbeard really.