I agree with several of the other answers that state that looking at many, many projects for mini-examples of what you want to do in your own app is the way to go.
However, you asked for an example of an app demonstrating best practices.
You could do worse than to read Matt Gallagher’s blog, Cocoa with Love from beginning to end. However, the app example you asked for is right here.
Not only will it show a variety of techniques, some novel design and best-practices, but also he points out where he feels that he might have done something better.
It’s a great read.