None.
If there's anybody in the division that could beat him it's either JDS (although he got mauled twice, I just think he has that power and his boxing is that good) or Barnett, since I think Barnett may be able to stop the TD's and he'd give Cain a good fight against the fence.
I do think Barnett would eventually slow down and Cain would really start beating him up though, I'd take Cain by UD, but I think Barnett would give him a tough fight.
Other than that, I would love to see Cain maul Travis Browne, it would be beautiful.