I don't know if it still is, but the original PC version of Cave Story for me. One of the very few PC games I love, as PC gaming wasn't part of my upbringing much. I have an old Windows 2000 tower I keep just to play it, even though I'm sure there are a billion betterways.

Also, I had some fun for a bit with Yume Nikki. Such an odd tone to that game.