Mine is Becky! Internet Mail https://www.rimarts.co.jp/becky.htm (sincere)

Great oldschool UI coupled with support for all the modern protocols, and consistently updated: https://www.rimarts.jp/downloads/B2/Readme-e.txt

Thunderbird still reigns supreme

Best one I've ever used was Fastmail's web mail. But it's locked to their platform, so yeah.

I use FairEmail on Android, which aggregates both my Gmail and self-hosted accounts.

I use Gmail's web interface for my Gmail account (which, I'll admit, makes my little rant above more than a little hypocritical)

And in my self hosted email setup I use RoundCube (web interface).

eM Client for various M365 and IMAP accounts. ElectronMail for the 9 ProtonMail accounts I have (allows "tabs" for each account in one client).

Mac/iOS Mail is pretty lightweight. I used to use Thunderbird on Linux.

EmClient, Mailcraft