Is there an overall best matrix client from a privacy and security perspective?
Most of those (if not all) are open-source, have good privacy policy, and I think are quite secure. You should look for UI, features, and usability.
I have ElementX on one phone, it looks nice, works well, but UI is terrible IMO. Have been using Schidli chat (Element fork) and it’s good. On PC, i use NeoChat (from KDE), it’s ok, but I use it only for public groups (mostly software chats). And the one I like most is Fluffy Chat, though they recently had some UI change that I don’t like that much.
Element is the best from a pure security and privacy perspective, as it uses the newer security stuff first (like going from libolm to vodozemac). It is the flagship client. Usability and UX wise…Element is fine but there are chances of weird UI bugs.
But as someone who used Discord forever and jumped ship, I have adapted well to Element. I like my E2EE messaging and decentralisation.
There’s always a browser tab in an isolated browser instance and/or private browser. I use Element’s web UI, but even that leaves a lot to be desired.
“Best matrix client” though is in the eye of the beholder.