With Strange it's always hard. You can't point to this power or that one...there's just lots of different spells.
The brains and brawn combo of Doom and Magneto seems great, but if no prep time (since it wasn't specified) hurts Strange from being fully prepared, then it also means that Doom can't have some magical or mechanical whoosamaboob on hand to help either. I don't know much about Doom's mystical powers, but if the suit helps him then fine...he'd have it for the fight, so even if it means that he alone isn't the 2nd best on earth at sorcery, he's still 2nd best with the suit...so he might be a sizable distraction if not quite a match for Strange.
I'd still go with the Sorceror Supreme here...but not by a lot.
-DM