Many questions in life are not so easy to answer. However, when it comes to what my favourite game was on the C-64, it is different. There is only one answer possible and those who know me long enough know it:
You can probably find the game these days on the various archives for emulators (there was also a version for the Commodore Amiga by the way). And some really crazy guys developed a Flash version. The controls are a bit sluggish, but the idea is really cool nevertheless!!!