Cave Story
Super fun retro gameplay, fantastic setting, lovable characters, beautiful chiptunes, along with the most rewarding (and punishing) secret ending I've ever encountered, all combine to create an experience I'll never forget. The word "masterpiece" very rarely applies to video games, but seeing how the entire game was crafted by one man (engine, graphics, story, music, everything), I think it's deserving of the term.
TUBM likes to travel?