There is no one best, as we wish there would be. Depends what you want.
XMPP and Matrix are definetly the most based ones, because you are not tying yourself to one particular app and server, they're the common languages. And this is what I would want to use for mass communication and as a base, default.
Signal is nice if those above are not enough developed yet for you. Easy to switch friends into and discover contacts with it's phonebook based nature. But there is no open API for thrid-party apps, only reverse engineering from open source code.
If you don't need calls Matrix has a bridge so you can use both at the same time.
There are also the most anonymous ones, like Briar, SimpleX or Session, there is a lot of them. For me their usage is when two or more people want really private chat and both agree on the app. I really can't and don't want to see them as the default.