I use all of them, just to clarify. Since they are hosted on Iceland and their laws regarding privacy are one of the best in the world that gave me trust, since they also meet the same security standards (or even better?) and since they have opened their source faster, are already developing an F-droid application, and they listen to you on their GitHub it seems like a good option.
CTemplar: https://www.hardenize.com/report/mail.ctemplar.com/1581655196
Protonmail:https://www.hardenize.com/report/mail.protonmail.com/1582127300
Tutanota: https://www.hardenize.com/report/mail.tutanota.com/1582127558