He's *one* of the most powerful cosmic villains in the Marvel U, but he's not THE most powerful
He's more powerful than most, if not all, Avengers villains. And with the Infinity Gauntlet he was
THE most powerful villain they or anyone else had ever faced.
nor is he the end-all, be-all, "ultimate" Avengers villain that some people here erroneously claim him to be. Hell, he's not generally associated with the Avengers specifically, but instead acts as an ubervillain to Marvel's more cosmic characters, like Adam Warlock, Silver Surfer, Guardians of the Galaxy and Infinity Watch.
He was never an enemy of the Infinity Watch. And no, he was never specifically an Avengers villain because he was bigger than that. He was a universal threat that everybody has to band together against in order to win. We're talking about a guy who gained infinite power three times. Captain Marvel had to call in the Avengers to help him beat Thanos. Thanos then went on to wipe out half the universe and the Silver Surfer had to call in ALL of Earth's heroes (what was left of them, anyway, as Thanos killed half of them, too) to take him on, and that still wasn't enough.
He is far more powerful than Loki, that's for sure, but there's still a ton of villains and groups in the Avengers rogues' gallery that can give the team one helluva fight for many sequels to come. Posters here are just exaggerating (and are just plain wrong) when they act like he needs to be saved for the climax, because (they believe, wrongly) that Avengers will have no more "big guns" to fight after him.
Name a bigger gun than Thanos, because that's the point. The whole purpose of storytelling is the concept of
rising action. You increase the stakes as you go. Things get harder. For example...
Avengers ---> Loki tries to conquer the world.
Avengers 2 ---> Ultron tries to wipe out humanity. Human extermination is worse than humans being conquered. The stakes are increased.
Avengers 3 ---> Thanos wants to wipe out all half the living beings in the universe. The genocide of half the universe is worse than the genocide of the human race. The stakes are increased even further.
That makes more sense then...
Avengers ---> Loki tries to conquer the world.
Avengers 2 ---> Thanos wants to wipe out all half the living beings in the universe.
Avengers 3 ---> Ultron wants to wipe out the human race.
...or...
Avengers ---> Loki tries to conquer the world.
Avengers 2 ---> Thanos wants to wipe out all half the living beings in the universe.
Avengers 3 ---> The Avengers battle it out over the SHRA.
In both those cases the stakes DROP from the 2nd to 3rd movie. That's
not how you tell a story. If the stakes in the 3rd act are less then they were in the 2nd act then the story's a failure.
The Avengers have plenty of villains which could be classed as big guns, but the fact is that there is no villain they have that can surpass Thanos.