Avon definitely got beat by Marlo. Marlo took his corners, his captain (Stringer didn't want a turf war, Avon didn't mind one), and then Avon becomes his proxy connect, talking about "Westside" unity. Marlo essentially crushed the Barksdale organization. The Barksdale name was pretty much done by season 5. Between McNulty, Omar, and Marlo, Avon was left with nothing but his name.
By the end of the show, even Marlo's reputation was fading. The king stays the king, but who plays the king is always changing. Its probably gonna be Kennard when he gets out of jail lol