I imagine this comes down to personal preference a lot, but I strongly prefer Roundcube over Gmail. They have improved a lot over the past years, if you have some preconceptions. After 10y+ on Gmail I finally feel like I am getting a grasp over my own email.

But then I also very much prefer a hierarchical/directory tree approach to organizing e-mail than labels&search as Gmail does it.

More similar UX-philosophy can be found in Mailpile[1] and Cypht[2]. Both still have decent amount of moving parts but are continuously progressing.

[0]: https://github.com/roundcube/roundcubemail/

[1]: https://github.com/mailpile/Mailpile

[2]: https://cypht.org/