I guess he is near the top, but for me he won't ever be anything else than a fine traditional medical ninja but a mediocre ninja otherwise. He's nothing but an Oro copycat.
From what I can see, Kyuubi is more wicked than the rest combined. And since when did Kabuto have experience with the tailed beasts anyway? Because he seems to be a villain who can pull off anything without an explanation? Oro was one of the sannin and spent every waking hour from his very youth doing dubious research. Madara is an ancient fucker who somehow lingered alive. The rest of the villains mostly can do their own specific stuff very well. Why should Kabuto make any difference? He seems to have been able to perfect the haxor resurrection technique, but you'd think that especially meant he had time for nothing else.