I use Thunderbird because it lets me manage my Gmail, syncs my contacts, calendars and tasks with Nextcloud and also has wonderful encryption support with Enigmail. I used to also use the RSS and Google contacts, calendar and tasks sync (via plugins) functionality of Thunderbird as well and it was great. Overall a very full-featured and stable client, has adapted well to my workflow over the years.
Perhaps my only gripes are bad dark GTK theme support in some calendar UI elements and some performance issues such as occasional short freezes and a long launch time. Haven't seen anything that comes close to feature parity so I deal with those. 😃
Edit: I also use ProtonMail web client for, well, my ProtonMail. 🙂