Worthy of the fame it garnered over the last 17 years, Scythe is an excellent megawad that you should definitely play if you are up to the challenge that the last few maps offer. Map28 (Run from it) was by far the worst map as it is damn near impossible to complete it without losing a fair amount of health, but the other 29 maps more than make up for it.
Map26 was particularly fun.
A set of fairly beginner but competent and challenging maps. Most of the architecture is symmetrical and made of tunnels and basic rooms, but thing placement is pretty well thought.
WARNING: in level 1, do NOT try to fall into the lava even if you see a secret door there! It will lead you into a corridor which has a bug! You will be blocked by an invisible wall, unable to escape!
A pretty complex and hard Heretic level. Beware that it has a timed Wings of Wrath quest near the beginning. I can also give you a hint that if you're lost near the beginning, there's a door hidden in the dark (I opened the editor when I was stuck like this). Unfortunately the level eventually ends up being really frustrating, so it gets a 3.