I have quite a few old games here too. For me, nothing in the world beats Doom and Doom II, especially when they've been ported to OpenGL (
http://www.doomsdayhq.com/jdoom.php). I'm also a fan of Descent (the first one), ported to Win32/OpenGL as well (
http://d1x.warpcore.org/). Back in these days they knew how to make great game music.