Apple’s stock apps on macOS (it’s pretty good, fast, stable, has almost all the features I need) and iOS (sucks but I haven’t found anything better), Thunderbird on Windows and GNU/Linux (sucks badly in terms of performance, but everything else is much worse in some regard).

Rainloop for the web, but I almost never use it - it’s a backup option for me, I’m mostly hosting it for friends and family.