Big fan of Dormammu, though most people think he's a magic character which is just off. He does know magic but he's an energy character more than anything else, like an ultra galactus and since no one can take him with energy they go after his weak spot being magic, so he got good with magic to nullify that weakness (superman, you could learn a lot from this). As far as ultimate threats the guy that can basically eat all matter in existence to gain more power is pretty up there. I'd like to see the rest of his race actually.
Clea is also awesome.