The gallery app that came with your phone is probably okay, but it’s nothing compared to Focus. Focus is still a very new app, but it’s already evolving to take advantage of everything Android has to offer. This app has a clean design that makes it easy to find your images by folder, but there’s also a cool tagging system.
If you want to show someone else a photo, you can lock the screen to it so they can’t poke around. Going a step further, there’s a private vault where you can put any photos you want to keep away from prying eyes. It even supports fingerprint unlocking on Android 6.0. It’s free with a $2.99 upgrade that adds the private vault, a dark theme, and custom tags.
Focus (free, $2.99 in-app upgrade)