Personally, I think XMPP with Omemo is the best you can use today. You have classic desktop clients for all operating systems (e.g. Gajim for Linux and Windows

Personally, I think XMPP with Omemo is the best you can use today. You have classic desktop clients for all operating systems (e.g. Gajim for Linux and Windows) and there are clients for android (conversations) and iOs (Chatsecure). Most of the clients (pc+mobile) are open source. What's more, you don't need to disclose your phone number and the privacy enabled by OMEMO is great without any usability nightmares. On top of that, the network is highly decentralized. There are many providers for free and paid accounts and you could also host your own server.