11. Iron Man
Tony Stark doesn't have superpowers, nor is he an expertly trained assassin with intricate hand-to-hand combat skills. What he is, however, is a genius, a tactician, and a strategist, who can outthink threats far more powerful than him - and that gives him the chance to defeat them.
Many would refer to Iron Man as the MCU's version of Batman, and honestly I can see why. Like DC's Dark Knight, Tony studies his enemies and prepares for them, ultimately coming up with a plan that will allow him to neutralize the threat. To bring those plans to life, he has his limitless technology, and state-of-the-art suits of armor - which he is always upgrading and perfecting throughout his years of heroics.
The Iron Man suit obviously increases Tony's physical strength, allowing him to take the fight to far more powerful opponents like Thanos, and even fellow Avengers Captain America and Thor, while the energy blasts and durability of the suit gives him a chance of defeating them. He even employed a super-sized, super-charged suit named the Hulkbuster to battle the Incredible Hulk.
Tony's technology makes him a force-to-be-reckoned with, but it's his mind that makes him powerfull. After all, Iron Man saved the universe from Thanos by outsmarting him. Enough said.