There is probably only one situation, maybe two, in which it matters which of a team's starting pitchers is considered to be its best starter. That situation is the first game of a postseason series. The possible second one is opening day, but since that is just one of 162 regular season games it's really not that important.
To me it's silly to fret about having "an ace", or "a number 1", whatever the definition of that is. Just find your five best starters and put them in the rotation. If there is an opportunity to add a pitcher who is better than at least one of those five then do so if the price is acceptable.