Cap gets my vote for the GOAT Avenger of the MCU, personally. He had the GOAT pep-talks and the best trilogy in the MCU hands-down. Winter Soldier is still a top 3 MCU movie to this day
If this poll were based on the comics, I would have voted for Thor instead off the strength of the Walt Simonson run. The movies made Thor Larry Holmes status in the end though
Spider-man is Marvel's GOAT individual character as a whole IMO, but I don't like Spider-man as an Avenger. I always felt like making Spider-man an Avenger goes against the 'everyman' nature of his character. The Avengers are all larger-than-life ballers. I like Spider-man because he's NOT a baller, he's just a regular dude that's broke and down on his luck.
Respect to Iron Man though, because without him there would be no MCU. I feel like Iron Man's movies fell off a little bit after the first one, but his importance to the MCU both in and out of universe can't be overstated.