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.

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'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