On my desktop: GNOME Evolution - but only because my university uses Outlook w/ Exchange (cringe) and the UI is slightly more tolerable than Thunderbird.
On my phone: I just use the baked-in Samsung Mail app.
In terms of provider, I used to use Gmail for my personal, but got tired of Big G scraping my correspondence. I tried Proton, but its integration story is a complete joke (you can upload your calendar and contacts but there’s no DAV support, their IMAP bridge is a non-standard-compliant dumpster fire that doesn’t work with half the clients I tried…) so I ended up on Fastmail.