On Android I use Conversations[1] (from F-Droid), on Linux I use gajim[2]. For me it works, for others, I have no clue. Gajim is available on macOS and Windows, too. I dunno if by default it has the OMEMO[3], and PGP plugin though. I think they are a must-have. Make sure that "XEP-0384: OMEMO Encryption" is supported by the server you pick.

[1] https://f-droid.org/en/packages/eu.siacs.conversations/

[2] https://gajim.org/download/

[3] OMEMO is an XMPP Extension Protocol (XEP) for secure multi-client end-to-end encryption based on Axolotl and PEP. Might want to go through https://dev.gajim.org/gajim/gajim-plugins/-/wikis/OmemoGajim....