It would have been very easy to do the predictable thing and place Ant-Man pretty low on this list simply because he was a bit of a silly character that few people took seriously. But the truth is that he's very much like DC's Aquaman in that his somewhat unusual abilities are actually really cool, making him far more powerful than you might initially think. Remember, he's single-handedly responsible for saving half the universe as he gave the Avengers a fighting chance to undo the damage that Thanos did, and that highlights how vital he is to the MCU.
Sure, Scott Lang might not have any powers outside of the suit and he might not be the most efficient fighter on the team either, but his connection to the Quantum Realm makes him powerful in other, more terrifying ways. This is, after all, the man who can turn himself into a giant. We're pretty sure that a punch from Giant-Man would sting a lot more than one from the Hulk because of the sheer force behind it. Yes, he can't sustain himself at that size, but in those first few moments, good luck with that!
Similarly, if he shrinks down to ant-size, he can do untold amounts of damage to his opponents, their weaponry, or suits. Remember how he almost killed Tony Stark by disconnecting the circuitry in the reactor in his heart? Yeah, we're glad Ant-Man is on our side.