Though Ant-Man is not normally thought of as being in the same league as Captain America, Iron Man, and other top-tier Avengers, he does have a unique advantage over his fellow superheroes that gives him an edge. While Paul Rudd's Scott Lang might be a reluctant hero at times, he is a powerful hero in his own right and was instrumental in the Avengers reversing the effects of Thanos' snap and thus defeating Thanos in Avengers: Endgame. His unique worth though goes beyond his Ant-Man powers and it's why he's such an asset.
Since the Avengers formed in 2012's The Avengers they've had a rolling roster of superheroes join their ranks, each with a variety of powers. Ant-Man's powers come from Hank Pym's Pym Particles which allow Ant-Man and the Wasp to shrink or grow by reducing (or increasing) mass along with density and superhuman strength. In some aspects, Ant-Man's superpowers make him potentially one of the most powerful heroes in the MCU, as the Pym Particles also give him control of time and space. This enables him to time travel through the Quantum Realm in some circumstances, as he did with the Avengers in Avengers Endgame.
Notwithstanding the power Ant-Man wields due to the Pym Particles, it's his unique power of being underestimated that has served him best over the years. His adversaries don't see him as a threat, or certainly not at the same level as the other Avengers. But this underestimation is why he often gains an advantage in his fights. In Ant-Man, Darren Cross saw him as a bumbling annoyance at best, and yet Scott’s willingness to make the sacrifice play allowed him to ultimately defeat Yellowjacket. Conversely, Hydra knew the danger that Captain America posed and so threw everything, including The Winter Solider, at him. Similarly, every villain from Whiplash to Ultron gauged the powers of Iron Man and scaled up their attacks accordingly. The concept of a tiny superhero such as Ant-Man running around though doesn't strike fear or a semblance of danger into the hearts and minds of the MCU's villains.
It's not just the villains who underestimated Ant-Man's powers and ability to hold his own. When Scott infiltrated the Avengers compound in Ant-Man, The Falcon was blasé towards him, and thus completely taken off-guard by Ant-Man's power. Similarly, the sight of Scott dozing in the back of Hawkeye's van doesn't give Captain America a lot of confidence in Captain America: Civil War, and yet Ant-Man turns out to be his MVP in the airport fight. Turning into Giant-Man and getting into Iron Man's armor are key moments in the battle that give Cap's team an edge. Again, in Avengers: Endgame, Ant-Man's plan to travel through time is initially met with derision, with the Avengers underestimating the plan that ultimately saves the day.
Ant-Man's jovial, carefree, and somewhat childish demeanor makes him easily dismissible as either a person to rely on or for adversaries to take seriously. Hank Pym is one of the few people to see through it and give Scott a chance. Ant-Man and the Wasp: Quantumania will pit Hope and Scott against Kang the Conqueror, and whether Kang also underestimates Ant-Man, or he sees him in the same light as the other Avengers, remains to be seen. Either way, Scott’s powers amount to more than just Pym Particles.
