Gmail's labeling system is hard to beat. In particular, any given email can be in several "folders" (a.k.a. labels) at the same time.

All other services I have tried out so far do not replicate this approach so that an email can only be in one folder at a time. Of course, you can copy an email into several folders but then they are physically different emails on the mail server.

Despite this, I switched to the German provider www.mailbox.org as they do not index or process your emails in any other way. And in Germany, the authorities still have a hard time getting access behind the scenes. Further, mailbox.org allows the creation of mail aliases and also temporary addresses. The service is 3 € per month which is totally acceptable.

I see mailbox.org as a main competitor of Protonmail and Posteo.

EDIT: additionally, I am using my own domains that are redirected to my mailbox.org account.