Throughout the Thanos, the Mad Titan, terrorized the galaxy from behind the scenes, manipulating his puppets to gain ultimate power in the form of the Infinity Stones. It took the entire Avengers roster, their allies, and a time heist to defeat him for good, with the effects of what happened still being felt as the MCU wraps up Phase 4.
During the Infinity Saga, multiple characters played various roles in heralding Thanos' defeat. However, some characters clearly had larger roles in this and contributed far more to his ultimate demise than others.
Nick Fury
Nick Fury may have only appeared briefly at the end of Avengers: Endgame (assuming it was the real Fury and not a Skrull), but he certainly played his part in defeating Thanos.
It was Nick Fury that brought the Avengers together originally to defeat Loki way back in The Avengers. He was the one that united the team and got conflicting personalities like Tony Stark and Steve Rogers to work together. And it was only thanks to the Avengers that Thanos lost. Without Fury, there wouldn't have been a team working together to beat the Mad Titan, giving him a much easier chance at success.
Hawkeye
Hawkeye may have been one of the meanest MCU characters at times, but he still played his part when it came to defeating Thanos.
After Thanos' warship arrives in the present day and blasts the Avengers mansion into rubble, Hawkeye wakes up next to the newly recreated Infinity Gauntlet and manages to get away from Thanos' minions with it. With the creatures clearly hunting for the gauntlet, if Hawkeye hadn't been there, Thanos would've gotten the stones once more and all life in the universe would've been lost.
Doctor Strange
With his ability to view alternate futures, Doctor Strange saw multiple outcomes of the Avengers' fight with Thanos, with only one possible way of victory.
With this in mind, he already saw Thanos' defeat and laid the groundwork for this defeat by giving him the Time Stone, which allowed him to originally win but continued to build up to his defeat. Not only that, but Doctor Strange was one of many sorcerers who undoubtedly ed forces to open up all those portals that brought the armies of the heroes to the battlefield to confront Thanos. All this went exactly according to the plan that the Sorcerer Supreme saw in his visions.
Captain Marvel
Captain Marvel is easily one of the most overpowered superheroes of all time, and it shows when she showed up to help defeat Thanos in the final battle.
As Thanos' ship is bombarding the battlefield, Captain Marvel arrives in a dramatic fashion. She then engaged Thanos himself when he was about to use the Infinity Stones once again, holding her own against him despite his overpowering might. He may have succeeded in destroying the universe at that moment if she hadn't intervened.
Ant-Man
Ant-Man got extremely lucky by being in the Quantum Realm during the Blip, for it was thanks to this incident that the Avengers were able to reverse "the snap."
Thanks to being inside the Quantum Realm, Ant-Man survives the Blip and winds up in the present day after traveling five years into the future. Because of this, he gets the idea to use Pym Particles and the Quantum Realm to travel through time and retrieve the Infinity Stones in the past so they can use them to bring everyone back to life. Without Ant-Man, the idea of time travel would've never happened, and everyone dead would've stayed dead.
Thor
Thor is one of the strongest Avengers in the comics, and this strength obviously carried over into the MCU. It's with this incredible strength that he seriously contributes to Thanos' defeat. Even though he wasn't as strong as the Mad Titan during the course of Avengers: Endgame, Thor still took on Thanos head-on, lasting a decent amount of time against him, both at the start of the battle and towards its end.
Thor also had both Stormbreaker and Mjolnir with him, giving him even more power and giving him an even bigger chance to defeat the Mad Titan in the film's climactic fashion, blowing a hole through the massive warship and destroying it.
Captain America
There are many character traits that define Captain America, all of which truly go into his contributions in defeating Thanos.
Right from the moment he learns it's possible to bring everyone back, Cap focuses solely on that mission, inspiring and leading the team into completing their task and saving the world once again. Not only that, but even when all hope seemed lost, Cap stood tall, facing down a massive army with only his broken shield. His bravery and endless hope and determination inspired all those around him, giving them a further drive for ultimate victory.
Black Widow
Black Widow clearly isn't the most powerful Avenger, but the team wouldn't have been able to win without her ultimate sacrifice.
While on Vormir, Black Widow and Hawkeye learn that one of them must be sacrificed in order to retrieve the Soul Stone. After a short fight between the two, it is ultimately Black Widow that takes the big plunge off the cliff, dying instantly upon impact, but allowing Hawkeye to walk away with the stone. Possibly the noblest sacrifice from one of the MCU's bravest heroes.
Hulk
Hulk is easily known for his brute strength, but it's his brains and his gamma-irradiated body that sealed his contribution to Thanos' defeat.
After the Infinity Stones were collected, the Avengers debated on who was going to be the one to wear the gauntlet and bring everyone back. Ultimately, Hulk is the one to go through with it, as the radiation emanated by the complete gauntlet is gamma, the radiation that essentially made the Hulk. He then uses the gauntlet to bring everyone lost in the Blip back to life, creating the army that is essential in fighting and defeating Thanos in the first place.
Iron Man
Without a doubt, Iron Man gets most of the credit when it comes to defeating Thanos for the last time.
Knowing that there was only one outcome in which the heroes win, Iron Man decides to sacrifice himself, stealing the Infinity Stones from Thanos before erasing him and his army from existence just like Thanos did to half of the universe five years earlier. Combine that with the fact that he was the one to figure out time travel, making the time heist possible in the first place, and it's no surprise who contributed the most to the end of the Mad Titan.
