The strongest Avenger was created by Ultron and is Vision, even though the Hulk seems to have a great deal of strength, Vision is overall very overpowered, with the most abilities, intelligence, strength, is almost at a cosmic level. Thor is also extremely strong, but also has a good deal of intelligence, where Hulk while in Hulk mode is usually not incredibly smart or stable, but as Dr. Banner is really intelligent.
So for the most strength, powers, intelligence, Vision has long been considered by many as one of the very most powerful comic book super heroes.
Iron Man is a completely ordinary human being with a lot of intelligence, his strength comes from his technological suit. Vision can actually change the nature of his form to the point that he can reach into a person's chest and crush their heart, he is not human, and is practically immortal and like some kind of super-powered ghost.
The Scarlet Witch has a tremendous amount of psychic power (she was originally Magneto's daughter I think!) and has been involved with some very big things going on, but physically she is not extremely strong, but her psychic powers have at times been tremendous.
I have all the books or comics that deal with strength stats and much more information, and numerous other books dealing with all these sorts of things, including lots of the original comics. Things change, but here is a general ranking from least strong to the very strongest, and "strength" there doesn't mean only physical strength, but also a balance of other powers and abilities which can give the character an advantage over others or lead to them being able to "beat" another character in a fight:
Lowest End
Black Widow
Hawkeye (Has at times taken forms with a great deal of strength)
Captain America
War Machine (With Suit, arguably physically stronger but not as intelligent)
Iron Man (With Suit)
Spiderman
Hulk
Wasp
Ant-Man 2
Ant-Man 1
Scarlet Witch
Doctor Strange
Captain Marvel
Thor
Vision
It seems almost a general rule in the cosmology of the Marvel Universe or its way of typically ranking things, some of the strongest beings are Robots, then "Gods", then Aliens, then Sorcerers, then Lesser Demons, then Mutants, then Scientists, then Super-Powered Humans, then Genetically Modified Humans, Modified Humans, then Super-Skilled Humans like Black Widow and Hawkeye, then ordinary humans, men, women, children.
A lot of these can have advantages over others, certain powers that give advantages or certain knowledge, skills, influence, friends, or intelligence that provide them with an edge.
What is interesting though is how Robots created by Aliens or Humans have often been more powerful than almost anything else, though the most powerful beings of all are Cosmic Beings like the Beyonder and stuff.