It depends on many things, such as a threat modeling, opsec, etc. In terms of privacy and security !simplex@lemmy.ml seems to be superior.

Several reasons to that:

• SimpleX doesn't have IDs, unlike Session. Which makes it more anonymous and private;
• Ofc things like E2E encryption, forward secrecy and others;
• Message mixing is and underrated feature, as well as content padding;
• It has amazing security features such as self-destruct passwords, and a couple of others;
• Can be self-hosted;
• No need for phone number;
• Leverage several 'accounts';

I have read their white paper, and is worth the time. Also, one of the episodes of the Opt Out podcast is with the SimpleX creator. I suggest listening. I personally liked the way he conceptualizes decentralization, and problematozes protocols.

I found SimpleX to be the best of all private messengers. Better than Session, Signal, XMPP, DeltaChat, and others. It is also more convenient than Briar and Threema.