As an EFnet admin: EFnet ;-) IRCnet is still around too with some very active chat channels (e.g. #worldchat). The networks are much smaller nowadays though.

libera.chat is quite lively. IRCnet, efnet, quakenet, etc. are still there but maybe not as active as in the past.

I went from DALnet to EFnet / Undernet. They're still near the top of Netsplit's list of Top 100 IRC Networks https://netsplit.de/networks/top100.php

EsperNet is on the list too. I remember spending a lot of time on there way back when.

Want to talk about a FLOSS project? Libera.Chat, OFTC, GIMPNet.

Want to just chat? Snoonet, Slashnet, Quakenet.

Want to just chat but want it to be technology focused? Darkscience, 2600net.

This is a shortlist of public networks I can recommend to a new/returning user; there are many smaller networks out there that you might discover organically with time.

Rizon. It isn’t quite as active, but there are a few good chans to chill in.