I had a difficult time narrowing things down, and even still, it's hard to pick based on level design alone.
PROFESSIONAL:
PC
- The training and beginnings of the Half-Life/Portal series. They all set up the 'stage' for the games in general so well, and I'm just speaking from a 'level' perspective. If I had to pick one, I'd probably say HL1: Opposing Force.
- Valley of the Jedi Tower in Jedi Knight. I rather like duels on bridges and high places in general, as you'll likely see in some other choices.
- Beginning of the Marine campaign in Alien Vs. Predator 2. While I liked playing as the Alien the best, the first levels for the Marines I seem to recall really set the mood, the vast and confined ominous spaces, and the like.
Atari 2600
- Yars' Revenge - all. It's pretty much just 1 level for the game (2 if you count the reworked enemy shield), but the design of the space itself is elegant, with 'your' side and the enemy side and the neutral zone of sorts in the middle.
NES
- Half the levels in Battletoads. They were just so crazy, and the Revolution in particular is still a neat take on adding "3D" to a 2D game.
- Start of Super Mario Bros. 2 (U.S.) - Starting off by falling!
- The 'retro' level in Kirby's Adventure. It's neat to see throwbacks in games, and this was the first I encountered.
SNES
- Most of 10,000 B.C. in Chrono Trigger. I liked the jump between the two aesthetics and how you traversed.
- The first few levels of Donkey Kong Country for the mood, and Mine Cart Madness for the 'gameplay' part of the level design.
- The start of Zelda: A Link to the Past. I'm not only a sucker for mood-setters, as you've notice, but for rain as well. It was nice starting off with the 'sneaking' feel of the layout.
Genesis
- Chemical Plant for Sonic 2 - rushing downwards WOOSH!
N64
- Dragon Cave and Deadly Fall for 1080 Snowboarding. Something about the drops, and in the first, the nighttime. I'm going to go ahead and lump Diesel Disaster and Sick with It from the Gamecube's 1080 Avalanche here too, though overall, the levels in the Gamecube version were better than the N64 version.
- The cylinder tracks in F-Zero X. It gets an absurd amount more points now that I own a 64DD with the expansion kit -- making my own levels, woo!
- The Caverns in Goldeneye 007. The singleplayer had a nice flow to it, and I found MP to be great too and didn't suffer from the "where am I?" syndrome the MP-specific levels did (though I generally think that of a lot of the older FPS MP levels - the ones pulled from SP levels I always found better - not as sure with modern FPS games since I haven't played one for both in a long time)
- Bowser's Castle in MarioKart 64, though really, any Bowser's Castle in just about any Mario game is typically fun.
- The Rock track from San Francisco Rush (though I enjoyed most of those tracks in general) and the second "trick" arena in Rush 2049. Jumps!
- Macbeth from Starfox 64, because of its generally rocky terrain. Macbeth in the original Starfox was neat too in its own way.
- Tall Tall Mountain in Super Mario 64, as I enjoyed feeling like I was 'scaling' the mountain. In that regard, levels like the Lost Coast in Half-Life 2 were neat too, though it's always a little awkward when in first-person.
Gamecube
- The Endor battle Rogue Leader - it really felt like I was living the movie at the time! Though I still recall wishing I had the true 3D movement I did from the Skyhook battle in Shadows of the Empire.
Wii
- Pretty much all of Super Paper Mario, since the main game mechanic was tied in a lot with the level design, though if I had to pick, I'd say the 8-bit World and the Underwhere world.
- Pretty much all of Lost in Shadow, again, because the main game mechanic tied in a lot with the level design. If I had to pick a part, I'd probably say the beginning area before you enter the tower proper.
Playstation
- The rooftop of Twisted Metal. I love high places.
COMMUNITY-MADE:
Since I've only really played community-made levels in Jedi Knight...
Atlantis & Morpheus: bug was a damn good level maker
Siege on Derra IV (though I liked the similarly-based SP level for the same reasons - scale!)
Purgatory (it was just so HUGE and so much to explore!)
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