Kyouraku, hands down.
He's laid back, amongst the absolute top tier power hitters, is very well respected and connected, more or less always does the right thing, truly cares for his subordinates and friends, and his vice captain is by far the hottest woman in SS.
Failing that, I'd go with Ukitake, for mostly the same reasons.
And, failing that, I'd go with Kenpachi. He's basically a more blood thirsty Kyouraku in all things that count.