matrix: despite the honourable efforts of instance maintainers, i actually think that client apps are the achilles heel of matrix. every app is missing features i would really rather have, even if the server supports them all. everything feels like a hack built on a hack, and its fragile ecosystem shows. i can't recommend it if you're moving from discord, but i welcome you to try it out and see if it works for you and your friends!
signal: if your primary use for discord is DMs and group chats, you may feel right at home with signal! i just chatted with my girlfriend for about four hours with webcam and screenshare, and it was a very pleasant experience! do note that the desktop app is rough around the edges; signal is a mobile app first.
mumble (in progress): i have a server i've kept as a voice chat fallback for discord for months- if i can convince my friends to Put Up With the huge interface downgrade, i'd love to test it on its strengths. its weaknesses are quite the blight, though. it's open-source, so i wonder could a third-party client claim superiority with a better UX and design.
fluxer:
stoat (ex. revolt): same "copying discord" comment from before applies. despite being open-source, it seems to sorely lack self-hosting support, which i consider a must-have for my future community chat app. it's failed to take hold of discord's marketshare since it launched, and i suspect there's a good reason for that.
teamspeak: even ignoring controversial military ties, i echo similar sentiments to mumble, except that teamspeak also fails by being closed-source. for my purposes, it's mumble but worse. at least it's a bit prettier, i guess...