Protonmail without a doubt. It's the easiest to secure and doesn't have any hidden security features that prevent a full takeover.
Avoid using major providers like gmail, yahoo and outlook.
UPDATE Oct 2024: Protonmail has started locking accounts on takeover. Although most can be unlocked easily, I've had a case of a permanent lock. Because of this, I can no longer recommend protonmail.
In the meantime I have found onionmail.com and mailfence.com . Onionmail is very good but keep in mind that accounts get deleted if not used for 3 months.