Madara (He's the leader of the strongest organization for god's sake)
Pain (Yes, I feel I've been overwhelmed by his godly powers)
Killer Bee/Raikage (He survived the whole of Hawk attacking him and his bro's just as strong)
Naruto (Well, the series is called Naruto after all)
Itachi (He was ill/hurt when he lost to Sasugay and I don't think anyone can beat Susanoo)
Minato (The 'god' of the strongest village)
Chouji (Do I need to say any more?)
In my opinion, I would say killer bee.
Mandara- Well the only defense here is that we seen killer bee fight and we haven't seen mandara fight. So until then killer bee is stronger.
Pain- Though pein(s) is god, He is only go when all 6 are together. Mr. 8 can fight well on his own. But then again the 6 bodies are part of pein kegakenkai. (sry for spelling). Though I still think that Mr. 8 can take out pein if he doesn't fight all 6 at once like J-man did.
Naruto- better hold on his bijuu
Itachi- dang....no defence for itachi....i guess it better to see how thier fight would end up than to speculate how it would end up.
Minato- same thing as itachi. But then again I never seen Minato go all out in a fight. Or even fake a fight like itachi did, since he's been dead. But if they were to fight i think Mr. 8 would...........
Chouji- ummm.....i don't think i need to explain this one.