Protonmail.
Hassle-free end-to-end encryption, a simple interface and very fast. In use, the encryption doesn’t intrude at all and it’s quite usable as an every day mail client.  No encryption is 100% secure but that’s no reason to not to make it harder for the bastards.
“…[ProtonMail’s] security measures are intense: end-to-end encryption and user authentication protocols so rigorous even the creators can’t read user emails.”
“ProtonMail hosts its servers in Switzerland… so all user data is protected by strict Swiss privacy laws.”
“A new email service being developed by a group from MIT and CERN promises to bring secure, encrypted email to the masses and keep sensitive information away from prying eyes.”