I really did like Kabuto throughout the entire run of the show, he may not be the strongest no- but he just is so threatening and sneaky that he's a cool character.
Pain's ideals were also good & reminded me a little bit of Light Yagami, in making a perfect but forced utopia out of fear. His fights with Jiraiya & the kyuubi really made me like him even more.
Can't say i'm an enormous Oro fan- mostly because he takes complete command of any situation, he also has sorta just stuck around far too long- he takes a note from Ganondorf's book of being a bit too difficult to kill.