I'm happy with fastmail. I haven't used Protonmail and have had some doubts about them overclaiming about end to end encryption and stuff like that, but they sound good too. The concept of privacy in email is problematic since a) if the person you are emailing uses gmail, then Google has a copy of your email's plain text no matter how much encryption your own provider uses. b) Even if the email content is encrypted, having the metadata intercepted can be just as invasive, c) even if encrypted, having an archived and authenticated copy of a message can be a big problem due to e.g. rubber hose cryptanalysis, d) for secure communication to exist at all, both people have to be quite security conscious, which isn't easy. Technical features like cryptography are of very little help with that.

There's a good movie "Citizenfour" about Edward Snowden, and I remember reading that when the producers needed to have a private conversation while working on the film, they would go outside and talk, leaving their phones in the office. A real privacy approach has to go well beyond using the right email provider.

I like that Fastmail has humans answering support tickets. That's already light years beyond anything like gmail. I don't know how Proton is about that. Maybe they can do it for paid plans. I don't see how they can do it sustainably for free plans but who knows. The main drawback of fastmail is that it is on the expensive side, but I use it so much that it doesn't sting as much.

If you just want cheap non-megacorp email for your own domains, I like mxroute.com. Their sticker prices can be kind of high, but they frequently have sales with super cheap plans.