Oda recently made a huge splash with the 1 billion beli bounty, and to be honest, I don't know where he can take it from there. 2 billion won't seem all that impressive to me anymore, even though it should.
Luffy (Pirate King. Even if he isnt the Pirate King there are many parallels with GDR), Blackbeard or possibly Dragon. Far out guesses would be Sabo, Kidd and Chopper. High bounties go to those who are viewed as dangerous by the WG. Fighting power is just one component of that.
Right now I think Dragon has the highest bounty since he's considered the "Worst Criminal in the World" or whatever. I don't know where to place it though since we don't know the bounties of the Yonko.