Edit: lemmynsfw.com has been uncensored instance wide so anyone may view it's contents without requiring an account! Might remove or at least un-pin this post later. Thank you to everyone who has participated in this thread so far.

The top ones currently (in no particular order):

!porn@lemmynsfw.com - All things porn
!porninfifteenseconds@lemmynsfw.com - The unofficial successor to /r/porninfifteenseconds!
!hentai@lemmynsfw.com - General Hentai Community (and successor to r/Hentai)
!gentlefemdom@lemmynsfw.com - For gentle female domination.
!aigen@lemmynsfw.com - For AI Generated Porn
!tentai@lemmynsfw.com - Tentacle Hentai
!futanari@lemmynsfw.com - Futanari Hentai
!nsfwworldbuilding@lemmynsfw.com - Not Safe For WorldBuilding
!rule34@lemmynsfw.com - Rule34 content