There's a facet of Xanatos that puts him into the top tier of villains: he never seeks revenge. At some point nearly every character in Gargoyles goes out looking to get revenge on someone who wronged them, but Xanatos just keeps on trying to make himself powerful and immortal and not letting himself get wrapped around the axle trying to destroy the main characters. Shredder, Lex Luthor, Voldemort, Joker... so many villains are defined by their obsessive fixation on their heroes. Not Xanatos, because he's too fucking cool to be bothered.