Angry Birds started off strictly for iOS. The iPhone changed Rovio's fate as an application and games developer and they have since ported it to all manner of devices. Think Android, iPad, Playstation Network and even the desktop browser. What's your favourite device to play it on? Let me know in the comments!
Personally I still like the iPhone version the best as I always have it on me. Pretty impressed with the iPad and Mac versions of the game just because the graphics look incredible!