2) Ultron
If there was ever a single enemy that could claim the title of nemesis to The Avengers, it would have to be Ultron. Boasting the ability to not only withstand blows from Thor’s hammer Mjolnir, but also access any technological port, Ultron has remained a thorn in the side of Earth’s Mightiest Heroes since his debut. Part of what makes this artificial intelligence so deadly is his ability to disappear into the online sphere, meaning even if his body is destroyed he always has a chance at coming back in some capacity.
Of course, destroying his body alone is a feat considering it comes constructed from the near-indestructible metal known as Adamantium, an aspect that sadly had to be replaced with Vibranium in 2015’s Avengers: Age of Ultron due to 20th Century Fox owning the rights to the former.
Regardless, casting actor James Spader in the role was a work of genius, but sadly the character’s evil machinations were toned down to befit something odd: a dark-take on Pinocchio. No longer was this AI a cold powerhouse, but instead a wisecracking entity that couldn’t even beat Captain America in combat.
Ultron’s plan also made no sense. He wanted to cause a cataclysmic event on Earth in order to wipe out humanity and save the planet, except such an event would also destroy everything else including energy sources, electric grids and computer networks: all things that Ultron will need in order to survive and grow. His death at the end only served to cement his legacy as yet another instance of wasted potential.