If you're talking PC only games, you can't do better than Heroes of Might and Magic 2 and 3. The charm, whimsy and music in those games are spot-on, and the gameplay is very engrossing and challenging. If you're talking PC games with console versions, my favourite is Psychonauts. I played it on the Xbox (though apparently the PC version is even better graphically) and from the moment I started I loved that game - the humor, storyline, music and art style were so engaging and the platforming gameplay was really fun and well-designed. I'm a huge Tim Schafer fan and I consider Psychonauts to be his best.