The base game is good, but the best part is definitely the mods and custom engines. Psych Engine just improves the game so much with its QOL changes and amazing mod support.

I've seen people compare this to osu!mania, but mania doesn't have mods like FNF. There are lots of great mods that revamp the menus, add new cutscenes and even new game mechanics. I believe that's what makes FNF stand out the most.

I haven't interacted with the mod community though, so I have no comment on that.