Opencritic currently has it at 82/100. http://opencritic.com/game/4214/splatoon-2 People should use Opencritic over Metacritic because they are more transparent, and you can blacklist any reviewer you don't like from the aggregate score.
IMO Splatoon released at 81 and slowly went to a 9.5 from all the updates. Looks like they are following the same formula with the sequel, so 82/100 is a good score.
Edit: Nintendolife gave it a 10/10 but that isn't showing up on Opencritic yet. This should balance out Slant Magazine's terrible score of 6/10 (They gave BotW 6/10 too).
The sentence below is false.
The sentence above is true.