Ultron v. Nimrod
Ultron is one of the Avenger's most terrifying foes.
The Original Nimrod was an unstoppable mechanical monster that threatened the end for the Marvel Mutants.
If these Maniacal Machines were to collide in combat, who would survive?
Winner takes on the Uncanny Avengers!
DOOM DEMANDS MORE NACHOS!
Ultron would just take over OG Nimrod with a thought. Dudebot took over the PHALANX.
Bastion would be a (somewhat) better match, but even then, he's still a robot and Ultron is really, really good at taking those over.
On top of that, I don't see how Nimrod damages Ultron in his usual adamantium form.