The evolution of the App Store icon from drawing utensils (pencil, brush and ruler) to transparent popsicle sticks is definitely the most interesting. Ask someone today what the A icon represents, and they would probably have no idea, or think something like building blocks.
Game Center is definitely the worst. The bubbles have never represented anything remotely intelligible. Multi-colored blobs equals games? If you say so, Apple.
I never liked that the App Store icon took the blueprint portion of the Xcode icon, but at the same time it made sense - designing and building apps in Xcode and the finished product in the App Store. Now Xcode uses the same lame elongated capsules.