If you ask me as a developer of my apps, of course, I'd say all my apps that I developed during 2013 with a special shout-out to my Quotegram iOS app (<a href="https://itunes.apple.com/us/app/quotegram/id705700846" rel="nofollow">https://itunes.apple.com/us/app/quotegram/id705700846</a>). It taught a bunch in terms of planning, marketing, interacting with bloggers and the whole 9-yard.
Philantro (<a href="https://www.philantro.com" rel="nofollow">https://www.philantro.com</a>) - working with nonprofits and in the so called "Civiltech" sphere has humbled and made me a more gifted programmer. It's the sort of transformation that can only come with coding for the heart vs the pocket. We're launching in a few weeks.