Built from the ground up with the goal of letting people easily communicate without having to worry about being overheard - Signal

There are many apps that pay lip service to security and privacy, but Signal was built from the ground up with the goal of letting people easily communicate without having to worry about being overheard. 

The Signal app is a complete phone and SMS client replacement (though it works just fine as a standalone app, too) for sending and receiving encrypted calls and messages.

 The app's look and feel continues to improve, too.