I would say Signal is the best like mainstream normie friendly option but if you want the most reliable privacy imo your better off with Briar. It's less convenient for sure, but it requires no phone number, has no centralized servers, can route messages over TOR, Bluetooth, LAN, etc. Messages are end to end encrypted peer to peer. And even when stored on your device are encrypted. Downside is you need the other person to have an android there is no iOS app. But upside is it has a flatpak!