The MCU has come a long way from introducing its character via its own origin movies. Now it's pitting super-geniuses, super soldiers, sorcerers, and gods against galactic threats. Each new enemy after the last has increased the stakes and by comparison, made the human enemies in the MCU feel like kittens in the face of tigers.
That isn't to say the pure human villains of MCU are obsolete now. In fact, they have proven that the best of them can be more interesting than generic aliens or Norse demons. Because like heroes, their lack of superpowers makes them rely on their intelligence and cunning. One of them even managed to break up the Avengers. Others have also done some admirable damages.
10 Obadiah Stane/Iron Monger
Out of all the human villains in the MCU, Obadiah Stane is one of the most incompetent. He mostly lived off Tony Stark's success and even in that venture, he did rather poorly. The best Obadiah Stane could do was to fund some insurgent groups and play as an arms dealer, maybe even trying to kill Tony Stark in the process.
Obadiah did manage to replicate Stark's earliest Iron Man suit and even snag some power source for it (again living off Stark's success), but Stark still managed to beat him even with a glaring handicap. Still, gotta give props to Stane for making the CIA proud with their proxy war tactics.
9 Yellowjacket
The Yellowjacket or Darren Cross is a younger Obadiah Stane but more competent and can actually go up against his mentor. That's a tall order since his mentor is none other than Hank Pym, one of Howard Stark's biggest intellectual rivals.
Darren Cross was able to build himself a suit that's actually a more effective and high-tech version of Pym's shrinking suit. While the fight with Scott Lang was less than grand due because it all took place in a living room but it was a fair and square battle.
8 Justin Hammer
Justin Hammer is just one of the villains of Iron Man 2 and was overshadowed by a bigger supervillain in the film. However, he was a big thorn on Iron Man's side and there's also the fact that he managed to replicate Obadiah Stane's success with the Iron Monger but tenfold.
To that end, Justin Hammer created several squadrons of automated war suits that proved to be a challenge for Tony Stark. His business acumen is also one of the driving forces of the second Iron Man film; it created enough conflict to contribute to Tony Stark's self-destruction.
7 Adrian Toomes
Speaking of enemies that Tony Stark inadvertently created, Adrian Toomes is one of them except he didn't pester Stark. Instead, he had to face Spider-Man which is fair game as Tony likely would have erased Toomes both in battle and in business-- he already did once anyway.
Adrian Toomes and his engineering know-how allowed home to craft reverse-engineered alien and Stark tech to fuel his own supervillain persona. While the Vulture appears like a lesser version of The Falcon in the MCU, Toomes gets additional praise for being independent and self-sufficient.
6 Whiplash
By far one of Tony Stark's most insane and cryptic villains, Whiplash is an amalgam of several comic book enemies. The filmmakers simply culminated those villains into a Mickey Rourke character that's more deranged than usual.
Heck, Whiplash was worth including for his tenacity and bravery alone. He faced Iron Man with nothing but some improvised electro-whips while also being topless. He practically had no armor during that encounter and still nearly killed Stark. That's probably where Stark got his philosophy about not deserving armor if you're helpless without it.
5 Mysterio
Yet another Spider-Man villain who was spurned and created by Stark, Mysterio is quite a handful. For one, he's one of the post-Endgame villains and someone Spider-Man had to face without Stark's mentorship.
Mysterio is more of a dollar-store version of Doctor Strange but his inventions still allowed him to create an actual Avenger-level threat. Additionally, he did what most other Spider-Man villains couldn't do, which is to hit Peter Parker below the belt by revealing his secret identity.
4 General Ross
He's not exactly a direct villain for the Avengers but his actions have cost them many opportunities and even the safety of others. More than anything, General Ross is one of the Hulk's most prevalent and destructive villains. General Ross, along with Emil Blonsky or the Abomination, is technically the first villain shown in the MCU.
What makes him even more special as an enemy is how he's a grey area. His disdain for the Hulk is understandable yet the measures he takes to enact that hatred are just too extreme. Also, since he's technically a government entity, the Avengers can't just take him out. At times, he's a conditional ally for the team.
3 Alexander Pierce
Hydra has proven many times over that the worst human enemies in the MCU are those who pose as allies. They are the ones who manage to infiltrate the underbelly of SHIELD and even the Avengers. As such, figures like Alexander Pierce are some of the most sinister problems for the Avengers.
The extent of the damage he caused contributed much to SHIELD's downfall and decommissioning. Even with leaders like Alexander Pierce gone, Hydra is still going strong and will remain a constant threat. Alexander Pierce represented that best since the Red Skull is unavailable and with Captain America gone.
2 Aldrich Killian
As one of the worst supervillains that owe their existence to Tony Stark, Aldrich Killian's climb to the top of the heap is nothing short of impressive. He was once a budding and hopeful intellectual whom Stark dismissed and bullied so he became one of his most dire enemies.
Aldrich Killian not only managed to surpass Obadiah Stane's proxy war machinations but also created a formula that turns normal humans into super soldiers. In hindsight, Killian could have made his own super soldier army if only the MCU's continuity allowed him to, but too bad he was only an Iron Man villain.
1 Helmut Zemo
Finally, Helmut Zemo. He's the most legendary Avenger villain with no superpowers at all. He did what even Loki or Ultron couldn't do: he defeated the Avengers. All Zemo did was introduce some closely guarded secrets like a wedge to the already large cracks within the Avengers.
It was Zemo's brilliance and opportunistic genius that made him an Avenger-level threat. Along the way, he also managed to kill a former Black Panther and allowed a purple alien to wipe out half the universe. Clearly, Thanos should have hired Zemo instead of Loki.