There is still no trace of Rank S in the game, so we will assume that Rank A is the highest. Here are the Ranks available in Luigi's Mansion 3 and how to get them at the end of the game.
Each rank is defined by the amount of money you've raised over your adventure.
Amounts are based on reports from some players, so this may not be exact.
- Rank A: from $70,000 to $81,000 and more (if we supposed that there isn't a Rank S)
- Rank B: from $30,000 to $62,000 (up to $70,000 if we want it logical)
- Rank C: Under $30,000
The only thing that Ranking do is that Luigi can restore the hotel to its full form (if your each Rank A), as shown above.