The number one starter or ace debate is fun, because the terminology is so malleable. “Ace” means different things to different people. Hell, it can mean different things to the same people.
For myself, an “ace” represents the top tier of starting pitchers in the league, but it can also be drilled down to a team level. All teams have an “ace.” It’s their best pitcher. Their number one starter. The guy who generally receives the ball on Opening Day. That’s their “ace.” Brad Keller was the Royals “ace” last year. Brad Keller is not an “ace” when discussing the top starters in the game.