It's not a ranking of who's best. It's a ranking of who had the best offseason, and the Thunder rates highly according to NBA.com.
NBA.com highly-respected David Aldridge ranked the league's teams 1 through 30 on their offseasons, and the Thunder is No. 3 behind only Golden State and Boston.
Aldridge's basis for the piece: "Is your team better now than at the end of last season? The ranking reflects the belief on whether, and how much, that is so."
Aldridge identified Billy Donovan as the Thunder's "key man" this season because of the head coach's "extremely difficult tightrope" of balancing the duo of Russell Westbrook and Paul George with the needs of the team:
"He’s proved more than capable of handling NBA egos and motivating people.