I don't think there is a best. There are many good options, signal, telegram when used with e2ee chats, are not set up by default. Groups in telegram are not e2ee. Session has a good concept, but message delivery is not reliable. They are making improvements to the infrastructure. However, the big problem is, how many of your contacts use it? I have most on telegram, followed by whatsapp, and some are on signal. I have couple of people I can communicate with on session, but it's just not worth it. I also use protonmail with a few people that also have protonmail and we communicate that way, because those mails are e2ee.