On mobile there are various options. The apps which are likely to be most secure are ones which have end-to-end encryption enabled by default and which can also be onion routed via Orbot. End-to-end encryption secures the content of the message and onion routing obscures the metadata, making it hard for a passive adversary to know who is communicating with who.
There have been a lot that have come and gone over the last few years. Signal is probably the most popular and has stood the test of time. Whichever app you choose, make sure to read their privacy policy and make double sure that they’ve passed an independent security audit by security professionals. If they haven’t, don’t trust them.
Thats not easily done without good funding. Also make sure the security professionals are actually any good. It’s pretty easy to be a “security professional” in countrys where it is common for companys to not care about security except for compliance reasons.
Also the most secure app isn’t worth a penny if your phones os and other apps are easy targets.