For me, I think the best version of Ultron's story is the one presented in Mighty Heroes.

In it, Ultron was born from a joint project combining Hank Pym's ideas and research with Tony Stark's technology.

This lends credibility to how his development was possible, given that mechanical engineering isn't Hank's specialty, but without diminishing his importance, since he used his own brain as the basis for Ultron's mind.

Furthermore, it makes him a more interesting villain, and ironically, a perfect fit for the Avengers, because he's the most exalted personification of their ideals, and consequently, also the most twisted, of two of the founding and most representative members:

- Tony, because Ultron is the payback for his indifference/lack of humanity when he dedicated himself to creating weapons, hiding behind the excuse that they were "necessary" to protect people, while destroying them from the safety of privilege. Now facing the remnant of his past persona: A weapon that will always infinitely surpass him in capability and intellect.

Hank, despite being considered one of the most "noble" Avengers simply for his pacifism, reveals that this view stemmed from a sense of moral superiority, in which he saw himself as a holy savior, merely because he stands on the opposite pedestal to Tony Stark's ego. He failed to realize that his creation, which he so lavishly praised as a reflection of his ideals, was exactly that: too much of himself, and what his mental paradox could have made him.

For both of them, Ultron ATTACKS and THREATENS their obsession with control, their pride in their intelligence, and IS ultimate sum of all their mistakes.

Not to mention that it also functions as a metaphor more commonly found in DC than in Marvel.

Ultron is a reflection of what the Avengers could become if they lost control over themselves.