Anything that properly implements the double ratchet protocol is probably fine bordering on ideal. XMPP's omemo is my preferred implementation and IM service.

Technically e2e encryption over email or literally any medium that can exchange text using AGE or PGP is also fine and fairly battle-tested.

Since none of those are peer to peer though, someone will always know you sent a message to someone else, but not what it entailed.

AFAIK there are no widespread p2p and e2e encrypted solutions, but maybe I'm just not plugged in to the right places