Nice try but Kenny wins. Ikkaku went all out against the fraccion of the sixth espada but Zaraki toyed with and beat the actual fifth espada. No competition here. Ikkaku is literally outclassed. He is believed to be and believes himself that he qualifies to be a captain and he might but he would definitely be the weakest one as his shunpo skill is low and has no kidou skills and his physical power is not on the level of most captains Zaraki least of all. Also he says he wishes to serve Kenpachi for life because HE IS STRONGER.
@taichokage: To be fair, Kenpachi had to use both Kendo and release his eye patch to win. While Ikkaku didn't even release his Bankai. That being said, their's an obvious difference in their power and kenpachi would take this rather easily at this point.
@rein: What do you mean? Ikkaku did release his bankai and still almost lost. And while Zaraki took his eyepatch off and used kendo, that is merely him not holding back not a special last resort ability. The moment he stopped holding back is the moment he won.