Apple Mail. It covers the bases, doesn’t try to cram too much into a single app, and doesn’t lean into badly aging UI trends.
I’ve tried Thunderbird a few times over the years (including post-redesign) and it’s always felt clunky and kinda awkward. Firefox somewhat outgrew that but Thunderbird never did.
If I were to use something else, it’d probably be MailMate which is a lot like Apple Mail but with even fewer frills and more optimization.
