This is absolutely the thing I've done that I'm proudest of. I went from knowing almost nothing about iOS developement to releasing a successful app in just a couple of months. When I arrived, the company had a really buggy phone gap solution and a broken web service. I fixed up the web service, added a few features to their admin interface - and of course designed and built the app with them. Helping them out with UX decisions along the way. Unfortunately the company no longer exists, so you can't try it out for yourself, but there is a website with my face on still.