I'm a really huge fan of Code Geass, as I'm sure it's painfully obvious already.
But anywho, it's definitely R2 that makes the series. Season 1 was great, but it's not until R2 that I can really say Geass struck me as masterpiece material. The real plot twists take place there, the character development gets even more amazing, and of course what I would deem as THE best ending I've ever seen in an anime.