The Countries and Capitals app allows the user to browse nearly every country across the globe and get additional information on each country.
Building this app helped me learn more about the structure of a more involved AngularJS app. The directory structure is much more modularized than just having the index.html, style.css, and app.js it one containing folder. Separation of concerns makes it much easier when building larger apps, and also severly limits the chance of scope conflicts.
I also got a chance to use Angular UI Bootstrap, specifically taking advantage of it's accordion directive.
This app was actually as challenging as it was fun to build. It's also an enjoyable app for me personally. I'm not a huge geography buff, but the relatively simple but sleek design makes it a joy to peruse.