Definitely Proton Mail. That being said, avoid using emails for secure communications as the standard was never meant to be secure. Sure you can use PGP but it has no forward secrecy, meaning if your private key ever gets compromised, not just one email will be cracked but every single one you've ever sent. That's ignoring the fact that your recipient almost certainly has never even heard of PGP to begin with. It's not quantum secure either and thus living on borrowed time. TLDR use Signal for communication whenever possible.