On-time.
Any of the most crowded places will tell you that they cannot check you in more than 15 minutes in advance.
This game-of-telephone has all too many people interpreting this to mean arrive 15 minutes in advance.
Here's what happens at Disney - assuming equal sized parties, if you arrive for you ADR before the other people with the same time slot, you get seated before them. But you arriving early to check-in will not get you seated before another group with an earlier ADR.
The best answer remains the same - on-time.