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.

I personally suggest Tuta (and I use it daily) over Proton. Several reasons:

Proton:

1. it is leaky in terms of social graph encryption. Sun Knudsen has a great video about it (https://youtu.be/GdDFUycXR_M&t=0)
2. had this case about the climate activist (https://www.theverge.com/2021/9/6/22659861/protonmail-swiss-court-order-french-climate-activist-arrest-identification). And since they position themselves as a privacy company, this looks disturbing.
3. I’d prefer a such a privacy oriented company to be more open to anonymous payment methods.

Overall, Proton seems like a little more privacy-conscious Gmail alternative.

Tuta

1. doesn’t use Google/Apple notification servers
2. encrypts more stuff than Proton

PS In both cases, emails are not end-to-end encrypted. Even though both are marketed with E2E encryption by default. Again, Sun Knudsen has a great video about the topic (https://youtu.be/G2Jh8bQ2wM8&t=501).

Also, as far as I remember, Proton is more expensive while having less features (the cheapest option) than Tuta.