Same here. I remember it being sort of a turning point where I was like--"that's it, this game is way too cool".
I'll map one from each episode:
I like the progression in this level and the long crooked walkway raising out of the nukage is still one of my strongest memories from Doom in the early days for me.
Doom Nirvana an essay
Regarding love of Slough of Despair and Mt. Erebus. My initial experience of both maps was similar, but I am going to focus on Mt. Erebus since it perhaps represents the holy grail of episode 3 and perhaps of Doom as a whole. The more I played episode 1 the more I loved getting lost in the levels and searching for one more secret to find. It felt like an eternity before I finally got my grubby paws on the registered version of the game, and by that point I was biting at the bit to continue the adventure.
One of the main things I loved about episode 1 was trying to get outside, the growing desire and increasing curiosity to discover any new areas of this world I had come to know so well and become so fond of; to get outside the limits of the confining walls and explore the outdoor areas beyond; and whenever I did via a secret, those were always the best moments for me. I used to always equate finding secrets/getting outside in doom as some sort of higher purpose in the game, as if it represented the ultimate goal, beyond the ordinary goals of killing monsters and exiting the level. It may sound strange, but it was for me somehow the next evolution in the adventure, of taking the whole experience to the next level. Romero set me up with this cherished relationship to the game. How was I to know someone totally different was going to have designed the other two episodes of Doom? :-/ Such is the difference between Sandy and Romero and the rude awakenings to follow.
When I was plopped in the middle of E3M6, the 'ultimate doom/hell experience' without finding an intricate system of secret passages ala E1M3 to get me there it was a let down. It wasn't really required that there be a secret to get there, but just that there be the sense of adventure and discovery to get there. At the time I didn't know why it was a let down, which made the whole experience even worse, because here I had attained what the game had spent so much time building up in my mind as the ultimate goal, and instead of a eureka moment, the whole thing felt cheap. I felt betrayed. So instead of the expected reaction of "Doom Nirvana Wow!" I was let down that I didn't get to discover this awesome outdoor area on my own; I didn't get outside via some super-secret passageway; there was no contrast of indoor dungeon to finding my way up to the light and outside. The game supplanted the best part of getting from Unholy Cathedral to Mt. Erebus with an intermission screen. If you had started E3M6 while still within the dungeon of Unholy Cathedral and you had to find your way out, then that would have been much more gratifying. Instead, you had no part in the transition from the one extreme environment to the other; it was completely incongruous. This gives me an idea... if any map were to be called Nirvana back in the day; well, I suppose half of it would be E3M6 with the other half some dungeon-keep leading up to the fiery surface. If I'd remake Nirvana for this project, I would build the level to represent what back then (and still today) would be the ultimate build up and pay off in the Doom experience.
Certain things really worked so well for me in Doom (like rising catwalks, or hidden nukage passageways that led to outdoor areas that felt forbidden and on the outer limits of the game [E1M6 secret nukage passage could be argued to be one of the greatest single secrets ever for this reason]). I think these things in E1 worked so well because they were obviously cool and new and they were beyond what you as a player expected from the game. They added a whole other element of depth on top of the already great fun of exploring areas you were meant to explore and killing baddies you were meant to kill in a fluid, dynamic and fast-paced environment.
It's the sense of discovery that Doom gave the player that made it so great. When I got to E3M6 I was robbed of that discovery. It would be analogous to starting a Wolfenstein 3D level in a room with 4 extra lives and 30 crowns. No game designer in their right mind would ever do this. It would cheapen all the rest of the secrets you worked so hard to find in hopes of finding the ultimate treasure, the ultimate secret (as a side note, it's kind of like drugs--you are always looking for the better high, the next level--the ultimate high, the ultimate secret [I don't do drugs, or recommend them, but the analogy seems to work]).
I still like E3M6, but now for different reasons than why I wanted to like it back then; now I like it because it has fun and frenetic gameplay and the hoardes of cacodemons match the epicness of the level. I just don't have those warm and fuzzy memories attached to it the way I do to the E3M3 firewall secret or many areas in House of Pain (dried up skin floor/pillars, first blue armor secret; BFG secret, pillars of tortured bodies in a blood chamber; or perhaps more presciently, the awesome secrets of E1).
I remember looking at images of episode 3 in the readme of shareware Doom and being like "I can't wait to discover those outdoor areas!!" and then having all my dreams dashed because I didn't get to discover a single one of them. I was just spoon fed all three of them (E3M1, E3M2, E3M6), like opened presents dropped in my lap two days before Christmas. I suppose E3M1 can be forgiven because it's a dramatic and logical way to introduce the episode, but when I saw that open hell-scape on the box art I thought it was going to be the final level of the game, not the first! See, that open-hell scape represented to me the ultimate goal and destination in the Doom adventure, what all the levels were working towards in the evolution of the exciting and atmospheric environments that contained you in bases and teased you with the eventual promise of going outside--the spiritual end to Doomguy's mission: to venture out onto the very blood-soaked surface of hell, an Inferno-scorched battleground beneath your feet and burning mountains stretching toward a fallen sky; the final stage to seek out and confront the final boss of all your nightmares. That, to me, represented Doom Nirvana. I was robbed of the best parts of the game: they came in precisely the wrong order or the intermediary ones required no work to find; there was to be no discovery of such exciting, new areas through exploring secrets or caverns that led you outside and there was to be no outdoor hell-scape for the final showdown.
I think all this illustrates why, when I did reach a level in Doom 2 called Nirvana, I was let down again by the gimmick that Map21 ended up being: if only the makers of the game actually knew what Doom Nirvana was. A field of medikits it is not. Oh, the irony....