Well, dormammu is not as powerfull than some say (=> Can't even try to fight the Cyclop-like baddies in his realm).
And Eclipso can possess other more or less easily. Possession is a big weakness for Marvel Characters.
But Eclipso fear sunlight, as vampires...
Well... Eclispo or Dormammu... I go with Eclipso, for the "possession" trick.