I’m far from a privacy expert, but here are some things that I have been considering while researching this for myself:

• Zero-knowledge encryption providers like Proton and Tuta are great for privacy at the expense of convenience, like possibly not being able to use common IMAP email clients. Proton has a bridge app for their paid plans that allows this on Windows and Mac but not Linux or mobile, and last time I checked, Tuta doesn’t have this on any platform. This means that your email can only be accessed from their client, and more importantly, if a bridge app is not available for your provider on your preferred platform, all your email will be stuck in your mailbox forever with no ability to archive locally.
• What are the privacy, security, and law enforcement/intelligence cooperation policies of the provider and country where the provider’s legal entity and IT infrastructure are located? If located in a country with bad policies, spying on your email is much easier, especially if it’s not a zero-knowledge encryption provider.
• While using the provider’s own domain for your address is better than using a custom domain, most providers appear to have a policy of making your address available to others after you leave them. Mailbox, Posteo, and Fastmail do this. One of the only providers I’ve found that has a policy of not doing this is Runbox.
• Does the provider have a good reputation for email deliverability? This is a tricky one that requires some research. First, look at the provider’s DMARC policies with a tool like DMARC Check Tool. Mailbox and Runbox appear to pass all tests, but Posteo and Fastmail fail the DMARC quarantine/reject policy test, which apparently makes it easier to spoof your email and could make your emails less likely to be delivered. Also search the web for comments on users’ experience with email delivery, like “<provider name> email delivery issues”, to find out what people have said.