Wrong!
- For all the story of the anime, Ash never had a direct battle against Team Rocket's leader, until BW. And honestly, it was too short for such an unprecedented time. A rematch is definitely in order, with Ash winning this time.
- Ash's lose at Kanto's League was embarrassing, so I think it has to be compensated. Besides, in Sinnoh it was revealed that to enter in the Champions' League, you have to win the Pokemon League of your homeland. In Ash's case, that's Kanto.
- Ash could have battle Lance in the Pokemon World Coronation Series, but he didn't. I think it's obvious they are saving this battle for another time. Maybe when Ash will battle the Elite 4 of Kanto.
- In order for Ash to truly be "the very best", Ash would definitely have to capture the strongest Pokemon ever: Mewtwo. That would be his ultimate challenge. Besides, considering Ash is probably the only human Mewtwo has truly ever bonded in his life, it doesn't seem to be another option.