It's all according to what Avengers team you are talking about. If you mean only of the Main Avengers Team, then I'd say Hawkeye...Not because he 'sucks' but simply because he would be the 'least powered' of the bunch, being only a very skilled human, with special arrows. Now if you expand this into including groups like the Secret Avengers...well then it would have to be the Eric O'Grady Ant-Man ......He's pretty innexperienced, and goofy to be an Avenger (though I like the character). Now if you expand it to include groups like The Great Lakes Avengers....then you could pretty much put that whole team into the 'sucky' category!
Whoever wrote this is a bad person because Squirrel Girl could kick all the Avengers' asses.