There is no best, because none of them cover every use case or threat model. However, these are worth considering:

• Matrix, if you don't mind minor meta-data leaks (reactions and avatars have not yet been moved to the encrypted channel, IIRC).
• XMPP with OMEMO, if all your contacts are technically skilled enough to manage the requisite clients, servers, and protocol extensions, or if they have a skilled admin to do it for them.
• Signal, if you don't mind linking a phone number to your account, can tolerate an ecosystem effectively married to Google, and accept the risks of a centralized service that can be attacked or shut down by someone with the right access or influence.

In situations where your safety depends on anonymity from the powerful or well-connected, I would instead look for a messaging system tailored for such things. (It would, of course, require giving up some convenient features that most of us expect from a general-purpose chat platform.)