Mastodon - my main social feed platform that first introduced me to the Fediverse in general.
Lemmy - my second main social feed platform that originally substituted Reddit from years ago.
Matrix protocol - communication platform I use to connect with users on the Lemmy instance I'm on
Peertube - would love to get an account going and use it more often but still don't know how but there's FediVideo.
Bookwyrm - Goodreads alternative that I signed up for that could use more work for a genuine reading tracker.
BONUS: my least favorite Fediverse platform lately
WordPress - because I used to run art blogs on there before I heard word about drama about the CEO of the corporation so I basically had to put out my last existing art blog...RIP.
Nextcloud is federated? First time I hear about that.
For me it's Lemmy, without a doubt. Never used Twitter, tried mastodon to see what it's all about, didn't like it.
Matrix seems decent, but nobody I know uses it, and finding useful groups is painful, especially on other instances (servers, whatever they call them).
Also, I am confused at why nextcloud is at the intersection of networking, music, and multimedia.
Yes it technically has a video viewer and music player, but I would be very surprised if any person in the world right now is genuinely using it to post that content to the fediverse social-network style.