There are a lot of apps out there 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. A recent update has greatly improved the app's look and feel, proving that security and usability don't have to be at odds. Droid App
If you are seeking an alternative app store because you find the Play Store overwhelming and difficult to search, then we have another solution to suggest. You could try an app that’s designed to improve the app discovery process and aid you in finding the content you want, but that still ultimately plugs into the Play Store to download and install apps and games. Droid App