Big "E"

It would definatly have to be a cosmic power/ abstract entity.  The only times he has ever been defeated was when he allowed it to happen.  If Thanos ever got up to full confidence and full power, no mortal and many immortals would never be able to defeat him.  Check out the series the end.

Thanos is definately the smartest.  He has forgotten more than Reed, Doom, Tony Stark, Prof X, Dr. Strange and Hank pym put together have ever learned.

Thanos will be back when Marvel needs a sales boost and decides to do a cross the the board crossover event.  Although this time I feel they may bring him back on the side of angels.

Back before Arnold became the Govenator, he was supposed to do a movie in wich he was to pass on the mantle to the Rock.