XMPP. Oh, wait, company based.

• Signal if you want phone number based signups.
• SimpleX Chat if you want to experience full privacy.
• Threema if you want basically the same as SimpleX Chat, but want to give them money first.

But XMPP would be the best option. Snikket is recommended for easy setup.

Self-hosted XMPP using OMEMO included? OMEMO are based on Signal, hence my question.