The Metal Men previously clashed with Ultron in STF #1836...
I enjoyed seeing the ultimate Ultron in the last couple of episodes of What If on Disney+, but one moment had me scratching my head. SPOILERS for the final episode follow:
When Thanos arrives with the Infinity Stones to collect the final one from Ultron, Ultron uses the mind stone to immediately slice him in half, instantly killing him. A cool and shocking moment to be sure - but why couldn't The Vision do the same in Avengers: Infinity War with his Mind Stone, instead of trying to destroy it? I know there was a comment about how the Stones have slightly different properties in different realities, so I suppose that's a loophole. Still, The Vision could have at least tried to use it in that way against Thanos.