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Thor is the Avenger that is canonically oldest and has been fighting the longest.
Cap is the one that was a “superhero” first, acting like an Avenger before any of the others were.
Tony is the one who first vocally accepted that he was on a team called “The Avengers”.
Carol is someone who was a superhero after Cap, and the original Ant-Man, but was the impetus for the “Avengers Initiative”, which spawned a team she still hasn’t really joined. She was someone called “Avenger”, but hardly the first one to be called that, so it means little. She’s not the first Avenger in any way.