- Matrix is for now (like Discord) more of "slack like" service alternative (but has potential to become more than that).

- Telegram might be the easiest to use where you likely find the most people but I disagree with a lot of things done by/around telegram.

- Threema (now open source) is as far as I know the most privacy focused one and by far my favorite. I like the way they are doing many thinks and they have many small goodies (like you can give a small "thump up/down" to messages in 1-to-1 chats from the message received notification to indicate agreement/acceptance). Or the way they handle accounts (separate from phone number and license, you can give people your Threema id without giving them your phone, the "trust levels", etc.). Or that they have a clear&clean business model. There are also a bunch of aspects which are useful if used in a e.g. business context like account revoke-ability, in person QR code based key exchange, fixed chat history etc.