Loki was the heavy in The Avengers (2012), so it's only fair that he's the supervillain equivalent of Diet Dr Pepper Cherry. But just because Hawkeye and Black Widow lack superpowers doesn't mean they're Diet Dr Peppers. I think they should be reassigned as Dr Pepper Cherrys, leaving Nick Fury, the head of S.H.I.E.L.D., as the sole Diet Dr Pepper.
But the only opinion that matters is Robert Downey Jr.'s. Correction: Robert Downey Jr.'s agent, because he's the one who negotiated for his client's name, and his client's name only, to be placed above the title in the contractual credits on The Avengers's one-sheet.
That guy or gal is the real superhero.