I saw this line in a new CBR post about a new IM granov 'Age of Ultron' cover image. It called Ultron the Avengers' "greatest foe"
It got me thinking, well, maybe Ultron isn't that, and this is all marketing hype, but if not Ultron, than whom?
With some it's easy: F4 - Doom; Captain America - Red Skull; Thor - Loki (arguably).
But the Avengers have had so many powerful enemies over the decades, is there ONE that stands out as the quintessential foe for the Avengers? (and please don't say "The Hood")
Throw out some ideas of who their greatest foe is, just for fun.