According to the Famitsu-interview Nomura gave after the Days-Ultimania was released he wasn't. Xemnas is the strongest, followed by Roxas when at full power (meaning two Keyblades + no power absorption from Xion), Xaldin and then Lexaeus.
Really? Meh, it must have been he was a final boss that he got so much emphasis, meanwhile, Xaldin and the others had to be watered down for obvious reasons (albeit Xalding being a bit of a hard fight :X).