What are some of the hardest Doom 1/2 WADs ever created?

I've been wondering what the community has decided to be the more challenging WADs ever released.

When I mean hardest, I mean actual difficulty - be it through monster placement, weapon placement, etc and not artifical difficulty, such as throwing a player in a box room with a Cyberdemon with no weapons and the only way to kill it is by fisting it to death(what a beautiful image in my mind I have created with that sentence).

Very recent thread has answers: http://www.doomworld.com/vb/doom-general/74232-hardest-and-or-most-difficult-map-and-orwad/

In addition to those "standard" slaughter events, may I suggest something... a little different.

I'm going to exercise a little self promotion here and suggest that Double Impact on the hardest settings is likely among the harder wads ever made for Doom 1.

Hard wad for Doom 1? The Floor Below (re-released as The Room Below in ESP megawad for Doom 2).

I remember Plutonia 2 being absolute hell.

Malcolm Sailor's Chord series (bite sized maps with very clever game play), Death Destiny maps, most Time of Death and gggmork maps. I would also recommend most of the maps in the thread that Sci pointed out.