I highly recommend the "Whispers of Satan" add-on campaign [0], played through a modern source port like Zandronum [1]. Beautiful level design [2]. Good difficulty curve. Clever secrets. Just a great overall set that uses the DooMII engine/monsters to maximum effect.

[0] http://www.paulcorfiatis.com/wos/HTML/wos_index.htm

[1] http://zandronum.com/

[2] http://www.paulcorfiatis.com/wos/HTML/wos_screens2.htm