1. Loved Hollywood Holocaust, the cinema thing (yay, I can go inside), the early secrets that were showed in the demo, the hydrant, the toilets, the movie, the entire level.
2. Loved Red Light District too. Strippers, adult bookstore, airplanes, building can be demolished, freakin' Octabrain!
3. Death Row: it was scary, frustrating and hard. I loved it. I didn't find the way out, but saw pigcops on the other side of the door with security camera, so there must be some way. And then, the submarine. How far did I get?
4. Toxic Dump, swimming, there can be more Octabrains on the map than 1, the crane, the shrinker puzzle (and not to die in it), more swimming (holy crap, why?), the gears. Again, how far is the nukebutton?
5. The Abyss. I got lost. This place is scary. Cacti hurt. I want more Pigcops, even they can't bear this place. Shrinker. More clueless wandering. Amazon in the secret!!! Ouch, long way down. That scary sound. And the Satan himself with a chaingun.
(after like 10 playthroughs)
6. Launch Facilty. Another level? REALLY? And I can blow up a rocket! (Wow, this poor level looked so good back then, just because of being a secret level).
Honestly, the demo episode was amazing. It's one of the best FPS adventures ever. Everything (almost) is so perfect. I very rarely found that excitement later on with this game, despite playing many maps. The five regular levels firmly belong to my top 20 levels of all time, some may be in my top 10.
After that:
7. Full version, demo of Dark Side. Wow, this thing is very good. More excitement. (Which wasn't followed eventually, but Dark Side, along with a few other levels like Fusion Station - dat reactor - were and are amazing).
8. Expansion pack. Wow, another episode out of blue. Thunderstorm in It's Impossible, and a girl broadcasts a weather forecast. And that tape self destructs! Did I mention the newbeasts in the end? It's Impossible was mind blowing back in those days.
9. Critical Mass and Derelict. Just triggering an atomic reactor to self destruct. How scary it can be? And that ceiling really fell. Then a big ship outside. Derelict. MORES STORM! And what a cool leveldesign it had. I loved it (although I have cheated in many parts, as it was hard).
Bonus 10. 25 years later we get a new mapset by the kings themselves. It wasn't exactly mindblowing, but Mirage Barrage and especially Golden Carnage were magic. Golden Carnage gave me vibes of 1996. That level legitimately looked like LA's awesome Meltdown.
And some usermap moments:
A. That Aqua series looked so good back then, maybe because the other maps in those packs were so bad.
B. Red3 by Merlijn van Oostrum. That level brought me into the Duke community. So if you hate me for my presence, blame Merlijn!

C. A Dream by Zyvkov Eddy... Well, a reasonable city map. all nice. OVER HERE! That harmless vent. What happened here? It was mind blowing. And somehow I returned into reality. But yeah, it's not the same place anymore. What are these big gaps in the shopping center?
D. Last Reaction and Water Bases. That was the TC. It made every other TC dull. That second episode felt like an official expansion. I was even thinking: did I pirate something? Those desolated drones in the final maps. And that boss!
E. Duke Forever 2013 TC. Seriously, this must have been made. And it was so satisfying. Not perfect (I'm still waiting for 2.0.), but lovely and adventurous. And a DLC had an awesome ride, enter Davox.
Nah, enough of ranting.