The Master's Levels for Doom II are the best of the best as far as Doom II add on levels are concerned. Several of them feature new graphics, sounds, and/or characters. They are made extra challenging, but do have different skill levels. If you're great at Doom II, you might be good at beating the Master's Levels. Requirements are the same as Doom II, you just need to know how to run an external level in Doom II.