Probably Blackbeard, partly because we did not see much of full power Yammy but also because Blackbeard is well suited for huge opponents. I could see this battle going how it went when Blackbeard fought giant Sengoku, given the similarity of his size, brute strength, and energy blasts to Yammy. Although Blackbeard didn't win that battle, he held his own and didn't actually lose. I would say that Blackbeard would at least hold his own, if not outright win.
yammy got pwned by two captains without real problem. thats usually not enough but heres my argument. He can absorb any ceros and even tank them. he would just quake yammy like nothing and tear him apart
He has both what is considered the most powerful logia fruit and the most powerful paramencia, he just claims to not have mastered them yet. And who knows what happened over the time skip. He is almost certainly more powerful now, possibly with even more stolen fruits and/or haki.