Ok so we can pretty much narrow it down to Luffy, Law, Kid and Zoro. Law has recently just been beasting the whole of Punk Hazard and Luffy is well... Luffy.
I mention Zoro even though he is said to be weaker than Luffy because he still hasn't even tried in any of his fights post time skip yet, his power is an unknown quantity which I think we'll see when he eventually fights Mihawk.
Kid is also an unknown quantity and his bounty is higher than any other supernova even though this is down to him killing civilians, surely there must be some substance in this?
Law has a pretty tremendous ability and has been shown to enforce it into his sword, I won't ruin anime watchers by what he recently did (haha) he also became a shikibukai and a serious player in DoFlamingo's weird underground devil fruit factory. Lets not forget what he did to Vergo... In my opinion he is the strongest supernova.
Red Hawk. Need I say anymore about Luffy?
So who do you think is the strongest?