A Major Doomworld Concern
E1M2: A deceptively short and simple layout for the second map, with a massive amount of hidden side areas just out of reach. Plus, it's got the lift trap room that completely changed the developers' perspective on what their game could be capable of.
E2M5: A thoroughly odd map, with no keys or other quests besides finding the exit, giving it a predominant focus on exploration. There's a lot to discover in this one. It's also one of the only levels in episode 2 to feature much of any height variation.
E3M7: So sue me. I like this one. It looks and feels like Hell, it has some good scale to it, and the relative quiet is ominous. The teleport puzzle never annoyed me too much, and finally solving it felt satisfying.
E4M6: Romero's final level for Doom, and a fitting swan song. Immersive atmosphere with plenty of memorable scenes all built around a central area, and lots of precarious catwalks to jump across. Visually and structurally, you can really tell how much he had gotten accustomed to building complex structures by this point, which is neat.
E1M9: A grid of brown box rooms connected by stairs. A pretty lame prize for the complicated secret exit in e1m3.
E2M3: I had a lot of "least favorite" choices to pick from for episodes 2 and 3. I'm going with E2M3 because it's flat, square, and thoroughly dull.
E3M2: Awesome! It's a big skeleton hand!!! And that, we find in E3M2, does not make a good map...
I'm not picking a least favorite in E4 because I don't really know all of it as well as the rest of the game, so I'm not going to pretend to know what I'm talking about to be able to complain about it.
I also tried picking favorites from each Doom 2 episode (01-11, 12-20, 21-30) but couldn't narrow it down far enough because some of its maps offer such different things from one another compared to the more homogenous Doom 1 episodes. What the hell, I'll just pick two from each.
E1 first pick - MAP09: The Pit
One of my favorite Sandy maps, with some absolutely surreal room connections and tricky side areas. More and more I'm finding this map to be what first comes to mind when I think of the definitive Doom 2 style.
E1 second pick - MAP11: 'O' of Destruction
A great-looking Romero map with a wide-open layout and the fantastic height-varied design his later maps are known for (or ought to be known for).
E2 first pick - MAP20: Gotcha!
Another deceptively simple Romero map, with a relatively short layout surrounded by a massive amount of secret areas to explore. Its scale, desolate appearance, and explorability make it feel like a lost world.
E2 second pick - MAP16: Suburbs
A fun, challenging Sandy level with a sandbox-style nonlinear layout that gives the player a lot of different ways to tackle it, most of which are supremely difficult. Demons have completely overrun the city, and it shows. The teleporting hordes suit this one really well.
E3 first pick - MAP29: The Living End
This one was an easy choice -- it's the penultimate map, and from the first seconds riding the elevator down to give you a moving view of the cavern, it feels like it. Some of the later areas feel comparatively a little uninspired after the first half, though that might just be because Romero set the bar so high to begin with.
E3 second pick - MAP24: The Chasm
The sheer scale of this Sandy map is terrifying; it does a great job of making the player feel tiny, and that's even before they get to the sequence of narrow ledges. While the indoor areas themselves tend to be surprisingly bland compared to the rest of it, the views looking out from them across the chasm are pretty awe-inspiring.