Going Down is one of my favorite Doom WADs. It has an imaginative premise that it takes full advantage of, making for a series of really fun, challenging levels. One of my favorite levels is Map 26, Insanity, which depicts Doom Guy having a mental break. Just really creative stuff. Some of the levels get a little too hard for me at times, but it doesn't change how much fun I have every time I play it.

Fun way to kill thirty minutes