I am in a slump. Here's some maps.
Containment Area - a Doom 2 tribute to Doom's E2M2. One of the great early Boom maps, this is enormous, very hard, and extremely detailed - indeed when I first saw it when it came out in 1999 (coincidentally at exactly the time DIY gained Boom map support) I thought it was the most detailed map ever, but it has been surpassed now. It uses all six keys and has exactly 666 monsters, which can't be a coincidence. However it takes a long time to play; the limited health and ammo, traps and ambushes require a cautious style of gameplay.
One irritation is the placement of the BFG, to access it you basically need to clear out the entire level, in particular, three of the four cyberdemons. I don't see the point in only getting to weapons or large powerups right at the end of a map when there's no purpose for them. However back in the day this wad taught me I could play cat and mouse with cyberdemons and kill them only with shotguns, so whatever. This is a true classic that everyone should play, especially if you like E2M2.
Insertion - 11 maps for Doom 2, very good old school style, medium to large. I played this in 2002 and can remember going on excitedly at people about it. It can be rather repetitive due to co-op-friendly design (e.g. having two different versions of the same area) Map10 "Hanford Disaster" is a great map except being annoying to play, with all those lifts. Map08 is fantastic too, watch the teleporter lighting and the supposedly empty plain! However, Map11, the ending is awful, a big grid of fast crusher puzzles is not fun - I probably could survive the whole thing if it weren't for this. Overall it's a decent wad if you like mid to late nineties large Doom 2 maps.
The Great Boss Battle Contest - Some contest on Newdoom. It produced seven maps. If you can get hold of it, play the winner, xbossmap.wad. It's one of those "let everything out at once and run round like a maniac" maps, great fun to play. Some of the others were pretty good too.