Signal is also very good
In comparison to protonmail, Signal has the added benefit of forward secrecy; it generates a new key for each new back and forth in the conversation. Net result: your encryption will be harder to break and less costly if you have your key stolen than messages sent with the same single key each time as with say PGP.
Signal is set up as a nonprofit funded by donations, in part to prevent the Whatsapp situation from happening after it was acquired by Facebook. As is, they aren’t looking to maximize profits and aren’t looking to sell. It’s a high functioning open source project with a thriving testing community. The CEO is also an anarchist, freighthopping vagabond who has a long track record of not giving a shit about money and giving a lot of shits about community and individual rights.
Signal also has great features: disappearing messages, sealed sender, and usernames instead of phone numbers are coming soon. Plus that good karma vibe.
As I mentioned above, Signal has amazingly strong encryption, which works to keep your conversations private and secure in transit. Your phone however, can most certainly be compromised, so the messages represent a liability as they are typed and once they are received and stored on your device.