full list of great villains here.
Honourable Mentions Surtur & Dormammu
Both are immensely powerful beings of pure malevolence. Both are cosmic level threats to the Marvel universe. Both would require a great deal of CGI to bring to life in film. Both seem to have a thing for fire. Surtur is a giant fire demon who is primarily an antagonist of Thor and Odin. He rules over the fire demons in the hellish realm of Muspelheim, at the farthest reaches of the nine realms. There he waits for the end of time when he will slay men and gods. He is one of the greatest threats to Asgard and the nine realms with his power equal to that of Odin. Dormammu is a being of mystical energy who is primarily an antagonist of Sorcerer Supreme Doctor Strange. He exists in his own Dark Dimension where he is a god tyrant. He is one of the most powerful mystical entities in the Marvel universe and is considered a threat to the life of the universe itself. Likelihood of appearing: Both Surtur and Dormammu have a very good likelihood of appearing in film. The casting call for Thor: The Dark World included a call for men skilled in fire breathing and fire twirling. It is likely that these men will be playing Surturs army of fire demons. No doubt these men will need their malevolent leader. It has been confirmed that a Doctor Strange film will be part of phase 3 of the Marvel Cinematic Universe. Dormammu is the primary antagonist of Doctor Strange so it is very likely that he will appear in that film, if not before then.