It comes from a 2004 interview with Yuji Naka conducted by IGN: Anyway, back on topic. My personal favorite special stages are Blue Spheres and the Sonic Simulator from Colors. Blue Spheres is really fun and gratifying once you get the hang of it, and Sonic Simulator is a nice bonus, doing a good job of giving you more gameplay and paying homage to earlier levels of the franchise. I also like how Sonic Simulator is completely optional. The other special stages aren't... bad, but a lot of them feel just like glorified tech demos. Sonic 1 and 2 in particular.