From what I understand, Tuta may have a slight edge theoretically, but email itself is a pretty poor protocol when it comes to privacy.
Tuta was forced by court order to implement a message logger for an individual, but AFAIK all of their previous messages were encrypted and could not be read by Tuta, and therefore the Government could only see new unencrypted messages coming in before they were encrypted.
Disroot only recently implemented at-rest encryption, so that should be fairly solid now. Posteo also allows you to encrypt your inbox and calendar at rest.
Even with that, consider all private email providers as mostly just to avoid surveillance capitalism (to prevent your data from being mined and sold), but with only marginal protection from state agents.
Offering a free tier let’s people try the service, and encourages them to become a paid user if they run up against the limits of the free tier.