• SimpleX is libre software and the most decentralised voice chat here.
• Briar is libre and the most decentralised but is missing voice chat.
• XMPP and Matrix are libre software and federated/decentralised. XMPP servers use far less resources, so creates more decentralisation than Matrix in practice.
• Signal is libre and more popular but centralised, acceptable.
• Avoid Threema's app which requires a service as a software substitute.

SimpleX

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.)

Where your friends are?

SimpleX is quite a promising project, uses Double Ratchet End-to-End-Encryption (from Signal), and has a very interesting protocol and model to provide quite strong metadata protection, especially in regards to whom you talk to and groups you're in.

If your threat model requires exceptionally strong Metadata protection, SimpleX is probably going to be your go-to

Though, for a more lenient threat model, where still good, but less laser-focused metadata protection is enough, Signal will probably do just fine.

Personally I use Signal, but I also have a SimpleX Profile, an XMPP Account and Matrix. (preferred in that order)

This spreadsheet is a very helpful comparison of the different messaging apps. I've been using SimpleX for quite some time now, and the only issue I have is some lag on the iOS client.

Depends on what you mean by "secure", being very loose with the definitions, we have

• end to end confidentiality (i.e. only you and the intended destination can see the message contents)
• privacy (only the destination knows i'm sending messages to them)
• anonymity (no one can find out who you are, where you live, i.e. metadata/identity/etc)

My personal preference is Simplex.

Reasoning for a few:

• Email: even if you use PGP to encrypt messages the server(s) in the delivery path have access to all metadata (sender, receiver, etc, etc). If no encryption is in use, they see everything. Encryption protocols in e-mail only protect the communication between client and server (or hop by hop for server to server)
• XMPP: similar reasoning to email. i.e. the server knows what you send to who. I should note that XMPP has more options for confidentiality of message content (PGP, OMEMO, others). So I find it preferable to email - but architecturally not too different.
• IRC: Again similar reasoning to email - even if your IRC server supports TLS, there is no end to end encryption to protect message contents. There were some solutions for message encryption/signing, but I've never seen them in the wild.
• Signal: Good protocol (privacy, confidentiality, etc). Dependency on phone number is a privacy concern for me. I think there are 3rd party servers/apps without the use of phone numbers.
• Simplex: Probably the strongest privacy protection you can find, but definitely not easy in terms of usability. The assumption is that we do not trust the intermediate server at all (and expose nothing to it), we just leave our encrypted messages there for the receiver to pick up later. It also does some funny stuff like padding messages with garbage.
• Matrix: In theory it supports end to end encryption in various scenarios, but my experience with it has been so bad (UX, broken encrypted sessions) I only use it for public groups.

Some more food for though though; these protocols support both group communication and 1-1 messaging - privacy expectations for these two are very different. For example I don't care too much about confidentiality in a group chat if there are 3000 people in there. It might be more concerned with concealing my phone/name/metadata.

In general I consider large group chats "public", I can try to be anonymous, but have no other expectations. e.g. some people use some protocols over ToR because they do not trust the service (or even the destination) but they try to protect their anonymity.

On a technical note: I don't think there is any protocol that supports multi-device without some kind of vulnerability in the past. So I would temper my expectations if using these protocols across devices.

I'm not familiar with the other ones that were mentioned in comments or in the spreadsheet.

For privacy it's probably one that nobody uses. Then it's even more private because you'll never send anything

You need to understand your threat model. Some apps are very secure but extremely inconvenient and hard to use. Others are more convenient but may not be able to hide the fact of a conversation between certain users for example

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.

"Best" is subjective. I like Signal because it has a great modern UI so I can pretty easily get non techy/privacy people using it.

I would recommend looking at this site. My personal recommendation would be simplex chat. It’s decentralized, doesn’t require a phone number and supports forward secrecy.

I'd go with Signal or Threema

Signal: Best data protection. They are on a different level from anyone else. They even reimplemented gif search through their app so it can be anonymised (instead of the data-collecting gif search in your keyboard). Just an example, they really try. Also has a desktop app that doesn't need the mobile app to be running. Downsides are google dependency (for push notifications - but they're empty, the encrypted data does never even touch google) and required linking to phone numbers. They do have usernames now so you don't have to give out your phone number to talk to someone. Behind it is an US based non profit - whether that is a downside everyone can decide for themselves.

Threema: No need for phone number, not even a credit card, you can buy it anonymously through their website. No google services required. Swiss based company, so much better laws than USA. Finance themselves through the one time fee of 2 USD and through their corporate offers, no nags for donations, no selling of data. Downsides are server code is not open source, and their protocol is less good than Signals, but still reasonably secure. They're working on a new one though. Also no independent desktop app yet (also working on it).

What is it you're looking for actually? "Best" is subjective to the person's needs.

For example for me, the best is signal but I would much rather use something decentralised that still allows friends and family to find me easily using my phone number. Stories / client defined groups without notifications are also very useful to me. Also a native desktop app (aka not a locked down browser running some local webpage) would be awesome. But such an app doesn't exist yet.

What would you need and prefer?

Anti Commercial-AI license

Delta Chat is quite good, it's an email client thats built like a messenger app. It's E2EE with Autocrypt lvl 1, you can use it with most email services, and they have a self hostable/hosted "chatmail" service that you can also use if regular email services are slowing down the messages (gmail isn't the best for this). It also supports apps and games in chat using the webXDC standard.

The other day I found this, using an very old inbuild command line tool in Windows, Mac and Unix: finger

https://happynetbox.com

Write for Example

finger zerush@happynetbox.com

in the command line

Email, probably. Kind of depends on your needs, and how willing other people are to accommodate them. The most secure messaging platform is email with a third party IMAP client using OpenPGP. That way the client and the server are run by different people, and the encryption is based on a verifiable and well known standard. But will other people use that to communicate with you? Probably not. So probably something like Signal would strike a good balance between privacy and ease of use.