The best time to be there is when the fewest day-tripper tourists are there (most people come during the day). So the best time is to be there as early as possible. But that means you probably can't see the whole park before it fills up. Most smart visitors arrive mid-afternoon, check into the hotel in the park itself (it's imperfect, but its location makes up for its flaws), do part of the park late afternoon, as everyone is leaving, then sleep in the park hotel, get up early, go do the rest of the park as soon as it's open, then leave by late morning when the crowds show up.
Tip: The same technique works well at most other places in Europe that are so popular they are being loved-to-death (there are many of those).