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I quite like Fastmail. I switched from Gmail five years ago and have had no complaints.I have heard fastmail has a rather good user interface. Gmail's is terribly slow these days.
They also are the only provider I know of that currently implements ARC (Authenticated Received Chain) http://arc-spec.org/ and are big proponents of JMAP https://jmap.io
If you're not bothered by the fact they operate in a FVEY country US/AU then they are an option too.
What I do like about Mailbox is they let me use my own domain (posteo.de does not).
They also implement DANE, MTA-STS, TLS-RPT. They use https://en.wikipedia.org/wiki/Open-Xchange which allows for things like Guard (temporary mailbox for recipients, they click a link write their message into the browser). They also have a @secure.mailbox.org alias that will force TLS, (not opportunistic).
Additionally they have inbox encryption which will encrypt anything you get in your inbox save in sent (via imap) with a sieve filter that uses your public key. You can keep the private one in your key ring locally.
ProtonMail has their bridge which is supposed to allow full body search (something which is kind of hard when you pgp crypt all your email), however I have read varying responses on the stability of that.
Tutanota has no API access, you must use their email client, web browser. Protonmail has contacts, but no calendars yet so you might still want to use something like https://www.etesync.com or https://radicale.org on your own server.
