Doom and Quake! When I used a potato PC as a daily driver the thing I played the most were boomer shooters. The source ports for both games are pretty amazing nowadays!
Awesome with the music set to OPL2 soundfonts on ye old soundblaster, or god forbid, a 3dFX Voodoo with an AWE 32. Don't forget to set your IRQ to 5 folks.