I've dabbled in many genres of game, and my favourites (call me immature or childish) include:
Goldeneye (N64)
Sonic the Hedgehog (Original, 2, 3 & Knuckles)
Pokemon (the main RPG series)
Super Mario Bros (The 2D platformers, especially World and New)
Command & Conquer Generals Zero Hour (PC)
Super Mario Kart (All versions, though I haven't played the arcade version)
While I find game programming to be challenging, demo coding can often be a way for me to focus on one or two effects or functions and explore them to the full without having to work on other parts of the program as much. I can then develop these features and use them in later projects. It's all very useful.