What are the most frustrating Doom levels?

[The Monitor]

Xaser said:

It still surprises me when I hear of regulars here who haven't cleared out or even completed every IWAD map (well, non-Final Doom anyways). It's pretty much required playing -- You're missing out. :P

Amen, brother.

The most frustrating to me, I think is MAP20: Central Processing of Evilution, irritating map.

[Tristan]

scalliano said:

Do the Master Levels qualify?

If we're including the Master Levels, then Bad Dream is a real pain in the ass to complete. Quite a fun challenge though, can't deny that.