On PC? Thunderbird, primarily. Its still a great, full featured client and I like supporting Mozilla. I also use Evolution and Kontact at times (on Linux of course). On Android, K9 Mail is a nice option.
I am a bit worried however about how so many webmail providers have started having "webmail only" kind of features. For instance, look at Gmail. Gmail has all those "multiple inboxes" that auto sort things like promotions, updates, priority mailbox, etc... but this isn't available via IMAP it seems since it isn't a "normal" sort of tagging. Am I right here? I'd like to have the same options using Gmail in Thunderbird as I would via the webUI. It used to be that Thunderbird didn't do OAuth or OTP style, but now it can thankfully (you used to have to get an application specific password).
On the other side, there are a lot of newly sprung up privacy focused mail providers (ie tutanota,
whiteout.io etc) that may use OpenPGP but to make it user friendly, it does all that stuff behind the scenes... making it so that you HAVE to use their webmail client and/or mobile app. Thankfully, ProtonMail is still the only one that gives you that option, but also lets you import or export the keys yourself, say..if you wanted to use Enigmail.
I really kind of worry that Thunderbird is not getting the kind of development time/power that it used to, since Mozilla backed off pretty much everything except Firefox and a few of their pet projects. Its still getting new features though which is nice, but I'd like to see it constantly updated to be a modern client with top tier PIM functionality and ensure that all open standards and whatnot arebeing implemented
