Kenpachi wouldn't have much chance against an Espada. He could take on any other Arrancar though. If he learned Shikai or even Ban Kai, that would be something to talk about.
Aizen's strength is unknown, but he is now the leader of the Arrancar, which would place him as the strongest opponent in the series. Kenpachi would be no match for him.
Zaraki has a huge potential though, his reiatsu is so huge, that even Ichigo felt uneasy around him. I hope he shows up soon in manga again. Along with Byakuya