It's basically just Signal if you want ease of use + good security. Not totally 100% since it is funded almost exclusively by the US govt, and I can't be sure if the encryption is not backdoored, but it's the best bet we got. IRC: not secure, XMPP / Matrix maybe ok but hard to use for most, Telegram wouldn't really trust though in theory has e2e, Whatsapp and Google world stuff even less faith. Honestly none of it is super great, but Signal has the best balance imo. There's also some crypto based messaging stuff that's used on darknets but that's the clunkiest
I think the only fully guaranteed method is having a pre shared one time pad encryption key between two parties & then send the encrypted text however you want (ex post on a far corner of a mostly dead online forum or Reddit). That doesn't have any fancy algos that may be bugged, or private/public key stuff