My favourite category is pacifist, without a doubt, since it is so diametrically opposed to the original purpose of the game. So some of my very favourite demos are the p1m7's, p1m5's, p3m1's, etc. of the Ultimate DooM.
I also have a tendency to thoroughly enjoy demos of something considered extremely hard being done the first time... for example t2m8, t4m6, 30nm, ty07, t4m1, n4m1... you get the general idea.
The most satisfying demos, though, are probably quick and brutal max-demos. There's something very special about "cleaning out" a map, especially if it's packed with monsters to begin with. I'm thinking av24/5 (Demonic Hordes), av32, hr22, hr24, pl32, etc.
I could be more specific about certain demos I like the most, and actually bother to answer the question, but I'm too lazy. ;-P
"One should never confuse the unlikely with the impossible." --John Dickson Carr