Inkademy: a major hub where beginners in competitive play can also seek helpful advice from seasoned veterans.
R/Splatoon: the discord server ran by the Reddit, semi-casual hang out mostly, useful for meeting people regardless of skill.
Splatoon Recruitment and Free Agency: Server where newcomers and veterans alike look for new teams to play competitively with.
Squid Colosseum: Server where Competitive players find scrims and league groups and such.
ThatSrb2DUDE's Discord: The Server ran by the popular Splatoon Player, ThatSrb2DUDE.
https://discord.gg/thatsrb2dude
SquidBoards Lounge: The server ran by the website, SquidBoards, also home to a major tournament.
Splatoon Discord: A large Splatoon Centered Discord
Pickup/Scrim Hub: A server for players to find other players to do scrimmages with or for teams to find opponents to scrimmage against
Turf War Server: A server for people hoping to find some more organized/competitive Turf War play
EndGameTV: Where EGTV events are hosted and announced
Splatoon 2 - Switch: A Splatoon 2 Centered Discord Catering to more casual players.
Splatoon 2 Discord: A large Splatoon 2 Centered Discord
Message Snek#9658 or Fuchsia#8801 on Discord for adding Hubs to this list