I have tried several, and liked none of them. I'm currently on Geary, but it's lacking in functionality, and it has things like search results being a bit different upon each of my searches. Starred messages cannot be shown on top. Eyeroll.

I think Evolution and Thunderbird are the top contenders, and of the self-hosted ones, Roundcube.

https://wiki.gnome.org/Apps/Geary

https://roundcube.net/