Liberius. Pope #36, and the first Pope not to be a saint. #37 through #49 were also sainted, so he must have been a bit of a loser.
Benedict IX. Liked being Pope so much that he did it three times.
Gregory XII. Holds the record for the most simultaneous antipopes. By 1409 there were three other claimants to the papacy, calling themselves Benedict XIII, John XXIII and Alexander V.
John XVI. Did not exist. They went from John XV to John XVII. So much for papal infallibility. No I am not making this up. Go look it up.