Any monster house at high levels is scary but I think the scariest room, or at least the one I dread the most...
Is...
The Greenhouse!!! Some call it a Fungus House, but I picture a room that has had so much guano/refuse/carnage left in it that all these molds and fungai start growing. Then throw in 30 or so levels below the earth and we start to see the death fungus. While I havn't seen one recently. I remember them fondly for murdering some of my best characters who were unfortunate enough to be piloted by me when my eyes were tired and not capable of distinguishing a grey period from a dark grey comma.
One thing is for sure. Moria/Angband loves them. Greenhouse levels are always superb.