Nostr is good at resisting suppression of public messages

It’s not really designed to be a great private messenger/DM architecture

I know some people were working on better DM stuff on nostr (nostr:npub1jlrs53pkdfjnts29kveljul2sm0actt6n8dxrrzqcersttvcuv3qdjynqn, others?)

Then there are different architectures/ideas like SimpleX, Keet, and maybe KeyChat coming soon.

What are the best DM/messaging tools today that fully open source, secure, and private?