First of I would like to state that these are my personal beliefs about who I believe is an underrated Marvel superhero. Also this does contain *SPOILERS*!
Scott Lang takes being a superhero as a new opportunity to create a better version of himself. Lang starts off as a criminal in the first movie by being in and out of jail and trying to raise his young daughter. Mr Hank Pym gives him a chance to shift his actions by making him the Ant-Man. Scott also rises as far as fighting alongside Captain America in the movie in Civil War and eventually becoming an Avenger.
Ant-Man knows how the Quantum Realm works and how the rest of the Avengers were brought back to fight Thanos, again. Without Lang’s knowledge of how the Quantum Realm works the Avengers that were lost would have stayed vanished and wouldn’t been brought back. Scott is very smart and with his creative thinking he has very good ideas to use in battle that can give him and his fellow heros an advantage. Also he created the idea to go back in time to save the rest of the Avengers.