Oh god, it's absolutely brilliant!
Hitman could just be another shooter game but I find it's best enjoyed when it's a little like dominoes - you know, set all the pieces up and just knock one down and enjoy the show.

I also remember the Theater level of R6:V2. Pretty good map.

Roof chase in Dark Messiah.
Vincent battles in Devil May Cry 3

Heh, so strange, after mentioning it I went through the lengthy process of installing and patching Bloodlines, got all the way to that level and played it though only to find it not as scary as I once thought. Then again since Condemned, I feel like I got an immunising shot against fear because of how brutal that game was; seriously I that game made me brick myself... though the level design was linear to say the least.

I vote for the Milkman level in Psychonauts.

Man, some of y'all really nailed some great ones.
I can't narrow it down to one, but I'm just gonna list some stand-outs.
I absolutely love that level.
I loved that one, too.
Omaha Beach from MOH:AA was another.
The chasing after the train in the nano-suit and fighting the enemy in Crysis: Warhead was awesome.
Loved the zero gravity levels in Crysis (original).
Pillar of Skulls and The Brothel from Planescape: Torment was awesome.
Virtual Reality level in Fallout 3 was another one I loved.
Geez, that's just some of 'em. Give me time to think. I'm sure there's more...

I think I'll run through this topic soon and edit my first post. Might give some idea of which level we think is the best. At a quick glance it looks like Shalebridge is in the lead.

While it is not really a level per say, I enjoyed two parts of Diablo 2: The fight towards the Cathedral in Hell, and the fight against the last of Baal's Minion's in the Throne of Destruction. Both show the best of the game (huge intense battles against powerful minions) and despite the difficulty, neither show any signs of letting up, and I would not have my Diablo any other way.
Fallout 3, my favorite level is when you return to Vault 101 and all has gone to hell, it just shows what a little event (or a big one in the Vault) can do to a small society.
In Dark Messiah, I like any level where you find yourself outnumbered and have to resort to using the environment to your advantage.
But my all time favorite level is in Metal Gear Solid 4, when you are in the bike chase. That has to be one of the best scripted events of '08.
*This list is subject to change*

"Facing Worlds" from Unreal Tournament (and the following iterations).
This level really blew me away the first time i saw it!
Capture the flag on an asteroid orbiting the Earth - Truly awesome idea!

I would SO have to agree with this. Totally freaked me out...

Hi guys, been on GoG a while, but my first post...
I've got a soft spot for the early levels of Syndicate (especially the first time you collected a crowd of people with the Persudatron - I think that was its name... - and had them following you as a human shield).
My top 10 list:
1) Guild Wars Prophecies (the first one) - the opening levels right up to and just after the Fall of Ascalon - this beautiful world you have been inhabiting totally ruined, and ripped apart. I loved that moment.
2) MOHAA - Omaha Beach - simply stunning and still a level I will go and replay
3) Call of Duty / CoD: UO - the last level of Pegasus Bridge, where you got slowly pushed back and back into the small concrete hut by the bridge, with the music masterfully swirling around you as the enemy advances on all sides.. and then BOOM! Allied fighters! It's one of the first times I jumped out of my chair during a game and cheered - I loved it! For United Offensive it's either the opening level (through the trees you see THE ENTIRE GERMAN ARMY!!!) or the WW2 action movie caper that is the British campaign in the expansion pack (taking out the clifftop guns, ending up in a boat chase out in the bay... so much fun!)
4) Doom - E1M1 - I remember ordering this game on 3 3.5" floppies - my Mum paid for it for me, we didn't even have a modem back then, and it was just a 386DX that could barely run it, but my God, that first room, when you looked through the window and you could see the mountains in the distance, and then the barrels exploded when you shot them, and the fireballs flew past you, and you could go up and down on platforms - wow! What a moment that was!
5) Doom - E1M2 - Two entries in the list I know, but the creepy flashing lights section with the Aliens-style wall decor and the imps hissing at you as you desperately tried to find a way out or the secret door to the chainsaw room... I can still picture it now - get yourself a copy of Ultimate Doom and the jDoom 3D engine and re-experience the way you remember it!
6) Homeworld - Missions 1 and 2 - incredibly hard, but the sheer beauty and complexity of this game was so well put forward by these first two missions, from the choral version of Canon in D, to the almost-surprised voice of Fleet Command - "We made it...".
7) Wing Commander 1 - Yes. The entire game. Loved it!
8) Half Life 2 - Nova Prospekt - Partially because of the fun factor returning after the hideous sand-buggy levels (I've never enjoyed the vehicle stuff in HL2 or the Episodes), but more because of the eerie level design, with those high vaulted ceilings, multiple levels for enemy snipers and hilariously a squad of Ant-lions backing you up!
9) Sam & Max Series 1 Episode 4 - with this episodic series I'll count the entire episode as a level, and this one is note-perfect. Just the thought of all the Mecha-Lincoln wisecracks is hilarious, but the bit that floors me every time (after that hilarious pre-credits joke with the voice bug) is when war is declared, and the Secret Service guys express their exhilaration through the medium of song, complete with OTT dance routines and silly in-jokes...
10) Portal - one of the few times I've actually enjoyed a Boss fight! And the payoff at the end is brilliant!
There are so many other fond memories - TIE Fighter being the first game I obsessively completed, Sonic on the Sega Master System just being so smooth and so much fun, the first time you free-run Altair up a tower in Assassin's Creed and then Leap of Faith of it, the Rogue Spear games and their obsessive planning, far more intricate than Rainbow 6 is now. Go back further, and we see F-19 Stealth Fighter, and keeping your radar profile low so that you popped up once, briefly, unleashed your bombs, and sped away. Or exiting the crashed ship for the first time in Unreal, getting your cape in City of Heroes, entering Hellfire Peninsula through the Dark Portal in WoW, winning your first space race in Civilzation,,,, I could go on, there are some really fond memories here!
P.S. Sorry for babbling on for so long - turned out I couldn't stop once I started thinking about it!

-"Naval Base", "Ocean Lab" and "Area 51" from Deus ex...well, really the whole game rocks.
-"Crazed Clockwork " from American Mcgee's Alice
-"Fourth floor of the Von Braun" System shock 2
-"Kashyyyk" the "Wookie Planet" from Knights of the Old Republic
-Londres Level from Tomb Raider III.

Halo-the final level...that last race against the clock is still kickass to this day
System Shock 2- The fourth level; just creepy 'nuff said.
Any level on Psychonauts
Grim Fandango- This level won't win any "best design awards" but three words: FLAMING DEMON BEAVERS!!!
Red Alert 2: Yuri's Revenge- The Hollywood level was fun because no other game allowed you play as the Terminator, Rambo, and Dirty Harry all in one level.

My god, really? I hated every step of the way... mostly because I despise wookies though :P
One of my favorite bits must be in the Test Chamber in Half-life 1 - the first time I played it through I had NO idea what was going to happen, and when it did I still didn't know what was going on! Awesome game that.