Gajim is the best Linux XMPP client in terms of end-to-end encryption support as it's able to speak OMEMO.
sudo apt-get install gajim gajim-omemo
Once you open it, you need to enable the plugin in the main program dropdown.
The only problem I've encountered so far is that OMEMO is not enabled by default, they made a PR but closed it because it encountered some errors that was not able to solve. It's a crucial feature, so if you have some spare time and know a bit of Python please try to fix it!
I've found the Developing section in the wiki to get started.
- Enable encryption by default: Nothing to do.