Adamantium Ultron wins. Both are extremely durable, but Ultrons encephelo beam will bypass Nefaria's toughness and attack his mind, Nefaria is only physical and as such I dont think could even hurt a fully Adamantium Ultron.
Nah, Simon was a bit resistant to the beam, but it was numerous beams from lesser Ultron drones that managed to take him down in the end. Nefaria wont be able to dent a fully Adamantium Ultron, and each encephalo shot will weaken him more and more.
And thats how Nefaria fights, throws opponents into space/sun? Nah, in character Nefaria will duke it out with Ultron and get frustrated he cant hurt him, while taking bolts to his brain til he falls. Both are massive powerhouses, but Ultron has a much more reliable and more used way to hurt Nefaria than the Count does back to him.