The default used to be a blog engine. It's a complex enough web app that you can learn how web servers and database access work in the language/framework. But i

The default used to be a blog engine. It's a complex enough web app that you can learn how web servers and database access work in the language/framework. But it's also easy enough that you don't need to spend a lot of time on a data model. The benefit is that the design of a blog web app is going to be very similar in a variety of languages that you can compare different frameworks/languages pretty easily. The downside is that if your language is better at non-webapp things, it's not going to be an easy exercise.