I'm in a unique spot to have both expertise in game design as well as complex video game programming experience.
My latest passion is Social gaming, and having worked over 3 years in this space (2 of which at Zynga) and having little to no social life during, has given me "expert" status in it.
The most impressive thing I had built was a full roller coaster tycoon style game for facebook. Think Farmville meets rollercoasters. Building and launching a game single handedly can be quite challenging, and even though the game only made a few hundred bucks in revenue, I still consider it to be a success in a grand learning experience.