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.)