The Destroyer armor is the armor was built to hold the power of Odin, Zeus and other gods, in order to fight Celestials sent to judge Earth. Once that battle had taken place, the life force of others animated the armor, at various times. These have included Loki, Thor and Dr. Doom. The armor, despite having been decapitated at one point, is supposed to be indestructible. If you picked one member of the Marvel Universe, who would you want to see take over the armor for good?

I think it would have to be a villain, and I can think of no one more horrifying than Ultron. If he couldn't be destroyed, inhabiting the armor, what kind of threat would he be to the human race then? (Of course, I'm not sure if his artificially created intelligence could animate the armor.)

What say you?

Anyone with a sufficiently strong will.

Magneto

I would say Tony Stark, but he's always upgrading his armors.... he wouldn't have the attention span to stay inside indefinitely. But then.... who really would? 
 
Shadow King.

@Tainted-Cell: I could see Tony doing it, if his means of keeping his heart going were suddenly ripped away from him.  That would drastically change the character of Iron Man, but I couldn't see him being happy with it, because he couldn't upgrade it. 
 
Shadow King would be an excellent choice!  Would it limit him too much though, compared to what he can do as an astral entity?  I really like that choice though.

id give it tyr god of war and let him run riot on anyone who thinks they can invade asgard !!!

The destroyer is built to protect the soul of the person wearing the armor making them invincible to damage. Ultron is not Spiritual, He's a Machine. A very deadly machine but one without a soul