The answer was MAP26
The question was a bit ambiguous, I suppose, more precisely it should have been "which is the only Doom 2 map that contains a solid hanging body you should be able to walk underneath" where being able to walk underneath is defined as being in a sector whose height (floor to ceiling distance) exceeds the sum of the heights of the hanging body itself and the player. The object in question is a hanging leg at (-704, 32) in sector 31; the sector height is 112, the leg height is 52 and the player height 56.
Doom doesn't contain any such objects; Ultimate Doom contains several, but only in episode 4.
The Vrack series
..are in some sense "unreviewables". They're far too complex and detailed to mention everything, and my writing a review of them won't influence anyone's decision to play them or not because anyone hanging around the Doom community online in the past few years would probably have heard of them and played them already. They are three maps set on space stations, apparently floating in a starry sky. Grey and silver and computer textures are common.
Vrack, the first, was a decent map, not too extreme in any aspects, its main distinguishing feature was that it was a space station map for Doom 2 at all, as Doom 2 especially is biased away from space stations in favour of bricks and towns. It plays very well - challenging, never dull, but not too difficult, and has a Hissy tribute (don't ask)
Vrack 2a and Vrack 2b kicked the whole thing up a notch. Larger, harder, and setting a new standard for the amount of detail in a wad. In fact if you want to blame any one map for all the idiots who refuse to play maps with normal levels of detail, it's probably this one! Also notable are the number of monsters, in particular the vast hordes of cacodemons that turn up at a few different places. It uses all six keys, has over 700 monsters, and goes on forever.
2b corrected a few bugs and added a reject table to try and speed the game up on older computers. Unfortunately it still wasn't enough for DIY on my Risc PC to cope, which is why I've only just got round to playing the map!
Vrack 3 finally appeared, quite a long time after Vrack 2 (and long enough after all the hype, including news articles on Doomworld about how the author "needs a new computer as his current one is not up to the task of editing Vrack 3", had died down) It was even bigger again, and still massively overdetailed but wasn't such a huge leap forward in that department. That's not to say it wasn't very, very impressive; just that there wasn't that much more you could do, Vrack 2 was already pushing the limits of what's possible with Boom. (I do want to mention the spinning holographic radar display though, one of the best uses of animated textures I've ever seen) Vrack 3 however suffers from annoying gameplay in a few places, there are a couple of traps which are viciously unfair and so forth. There's a maze made up by laser beams (instant death on touch) through which you are chased by a horde of revenants and archviles which teleport around making them impossible to shoot. This one needs savegames, which lowers its replay value.
2b is definitely the best one. 1 is a great map but somewhat overshadowed by others. 3, as I said, suffers from gameplay issues; I want to play it again, but I know I'll die at one of the traps and it'll just be a big frustration.