Spader's Ultron is, in my opinion, one of the absolute best voice casting decisions ever. Spader absolutely nails the disinterested, calculating, "I really don't care if you live or die but I'll obliterate you for opposing me" attitude of a deadly artificial intelligence hell-bent on eliminating humans. His performance gave me goosebumps in the original movie.
Spader is a great actor, and has been excellent in many things, but this will always be my favorite performance of his.