I would have to say Magellan fruit the doku doku or poison poison fruit or venom venom fruit what ever you want to call it that would have to be the most op fruit out there. No one could even get near you without the fear or being poisoned. You become immune to any physical melee attack, and just the very poison gas from your breath will weaken the enemy. Think about it Magellan defeated black beards whole crew with a single attack, they all would have died if it wasn't for the antitode that they were given. Also think about this out of every one that they have ever fought if I recall correctly that is the only guy that they never defeated they ran away from him even having such strong people with them.