The power scales got very bad near the end. By just logic, Bankai Zaraki should have been WAY stronger than what was shown. With his shikai he bursted a ****ing meteor, and defeated Gremmy (who was the strongest quincy besides the stern litcher). Without his eyepatch and no Shikai his reiatsu was already on bankai level. With Bankai he should have shattered the sky itself. But meh, tite rushed the end.
Bankai Kenpachi >> The last Ichigo we saw, who could only make his sword break, and break, and keep breaking.