Depending on your needs, XMPP or Matrix are probably your best bet. Both have different clients of varying usability and quality on different platforms, so you have to pick your poison. If E2EE is important, you also need to determine how encrypted you want your messages to be (as both XMPP and Matrix carry quite a bit of identifying metadata in its unencrypted headers).
For most people and use cases, either will probably do, but if you're a human rights activist or journalist in an oppressive country, I'd stick to Signal.
For most people and use cases, either will probably do, but if you're a human rights activist or journalist in an oppressive country, I'd stick to Signal.