Baron's Den beats out Ballistyx for the two pre-Mount Pain maps, IMO. There's this great area where, after doing a quick puzzle to get into it, I'm immediately bombarded by two Revenants, a Pain Elemental, and some nasty hitscanners perched way above me. This may not seem like much, but having to dodge homing missiles gives the Elemental enough time to spawn a score of Lost Souls, and so I'm running for what little cover I have while trying to deal with long-range melee and an influx of Cacodemons now appearing. Events like this normally tick me off, but there's no instant death involved like most traps of this typethe mapper trusted that this combination would put me within a hair's length of death without utterly destroying me. And that sets a good precedent for the rest of the map, which is either as challenging or less, but never compromised by needless frustration.
So that's a great map, with good variance between open areas, tunnels, and lava traversal. Ballistyx is more of the tunneling and nukage and lava, nothing else applies. And so it feels more linear without really being cinematic, which sort of sucks. More monster closets show up than in the last map, and there's a cool teleport that puts me in a pit surrounded by Hell Knights, Revenants and Imps all within range to kill or be killed; one great piece of imagery is a black expanse filled with burning flames, which players face when having to dispose of some Lost Souls or some other type. Solid stuff overall.