for me it's xmpp. now that monal on ios has almost reached feature parity with conversations on android, there's no reason xmpp shouldn't be the go-to alternative to whatsapp.
I don't think signal is the answer. a centralised service susceptible to all the things wrong with whatsapp. matrix is bloated. push notifications on simplex android is still sketchy.
and i dont buy the argument that onboarding is too complicated these days. most people can make an account for anything they feel is worth it.