Bubble Bobble. Definitely. Most of the games I loved for the C64 have aged badly, but not Bubble Bobble. I were playing Miami Vice again a while ago, and it struck me that, well, it's not really fun anymore. Same with a few other titles that I used to think of as timeless classics.
But Bubble Bobble is still as great as it was back then, so play that. And Stix. Bruce Lee, perhaps? Giana Sisters?