What are the most memorable levels/sections/tracks/moments? What makes them particularly fun, ott, clever?

****SPOILERS****
Obviously, but make the title of the game clear and people can read the bits they want. Here's some:

Tomb Raider: Anniversary

St. Francis Folly – Tomb Raider at its best – exploring a huge old ruin with very little combat and no interruption. After the first part you enter a large square chamber which just seems to go down forever. Finding your way to the bottom involves some of the finest ledge grabbing, switch throwing platform puzzling ever designed.

Yoshi's Island

Touch Fuzzy, Get Dizzy – Nintendo exploiting its SNES technology with a ton of clever ideas. This one’s a simple enough platform level, which is filled with these little fluffy clouds. Not too troublesome until you accidentally make contact with one and then the whole scene changes. Yoshi starts stumbling around drunk while the landscape warps around him, turning flat ground into hills and making each jump all the more troublesome.

Contra 3

Level  4 – Ups the already considerable scale and pace of earlier levels by throwing you in to a forced scrolling section starting on some sort of hover bike. You battle through a tunnel against flying enemies before being spat out to face a giant alien ship. You work your way along its underside against all manner of weapons before it flies off, and then things really get going. Finally a barrage of missiles is launched against the alien ship with you hanging from one. Just as each is about to make impact you have to jump to another, all the while trying to take down the ship’s defences and expose the weak point. Audacious and frenetic.

Dark Souls

The second bell - more a string of levels this, but it's the way they work together to create the sense of a descent into greater and greater decay and depravity. Having been told at the beginning of the game to ring two bells, one above, one down below, most players will find the higher one first, perhaps predictably in a church tower. So why would there be another down below? You start what turns out to be a seemingly never ending descent. What you uncover is a history, without any cut scenes or much dialogue, or any kind of scanning device or abandoned diaries. It’s just there in the architecture, the monsters and characters, and the level design. And each part is that bit more disturbing than the last.  

Bayonetta

The final battle against Jeanne - an epic encounter with an evenly matched adversary that starts atop a skyscraper, which then collapses as the fight continues with both characters ending up on the side of the building still fighting. Jeanne starts to summon huge missiles towards you which you can catch and throw back at her, before finally you both jump on top of one and the battle carries on. You finally end up hopping off the missile and onto the plaza of another building and fighting around a large fountain. The fight never lets up and the breathless action matches the spectacle perfectly.

Dead Space: Extraction

The first chapter – what starts off as a tutorial turns out to be a brilliant piece of narrative and sets the tone for a game that messes with your ideas of being in control of a character, especially in a linear environment like the on rails shooter. Playing as a technician working as part of a team responsible for shifting this large alien artefact, things start off normally with you riveting panels in place and operating consoles. Then things go wrong and people start to turn crazy, attacking you and your team mates. The deeper you get the more everyone loses it and the more people you have no choice but to kill. All the while there’s these weird whispering voices and strange symbols appearing around you. The ending ties it all together perfectly.

I'm gonna have to try Tomb Raider Anniversary at some point if the new one disappoints.  I only ever played the first one, past demo disk levels anyway, and thought it was absolutely superb. 

Micro Machines II

EPIC SPONGE BATTLE LEVEL.  Probably the best four-player experience ever.  Might have been called Stinky Sinks or something.  Second cup cake to the right and straight on 'til morning.

Edit: Actually, I think it was called Ferry Fiasco.

God of War

The moment when you fight through the desert and blow the special horn thingy. Through the dust ahead lumbers the biggest thing I had ever seen in a game at that point. Proceeding onto the back of Kronos and then entering into the massive temple on his back was just insanely epic.

God of War 3

Fighting the titan mentioned above.

Resident Evil 4

Approaching the first village, unsure of what to expect and then panicking as the shit well and truely hits the fan. Taking cover in a nearby building you take stock, check ammo, herbs etc. Then you hear the revving of a chainsaw and a classic is born.

Rainbow Six Vegas 2

This is more of a personal amazing moment. I was playing campaign co-op with my brother and I was in total contol of everything. We came to a room with three hostages and four enemies inside. I directed my brother to one door, took the other myself, marked the enemies and then led the countdown. We breached and took the enemies out in about a second. It was just perfect and so immensly satisfying.

Dark Soldier wrote:

Baby Park - Mario Kart Double Dash: Sheer MP perfection. Chaos, a sense of proper rivalry, and hours of fun after fun. Glorious, so much so I have an MK:DD tournament planned for next month.

Fuck off m8.

That's the worst one. My flatmate insists on only ever playing this level when I occasionally let him put DD on.

DD is shit anyway.

Frosty wrote:

Baby Park - Mario Kart Double Dash: Sheer MP perfection. Chaos, a sense of proper rivalry, and hours of fun after fun. Glorious, so much so I have an MK:DD tournament planned for next month.

Fuck off m8. That's the worst one. My flatmate insists on only ever playing this level when I occasionally let him put DD on. DD is shit anyway.

Nah m8 ur rong m7 I've played 4player MP on it for years and its fucking fun, as is doing 150cc all cup tour tourney but m8 dont h8 me m8 cos ur flatmate is silly

luv m8

Half-Life 2 Nova Prospekt. Loved sitting back and watching an infinite army of bugs dudes fuck my enemies up, but still being useful by taking out sentry guns and the like.

Devil May Cry 3 The bit where Virgil let him self fall into the abyss rather than let Dante save him genuinely bought a tear to my eye.

Resident Evil 2 Pretty much any appearance of Mr.X, he is terrifying and awesome.

Final Fantasy Tactics: War of the Lions Getting in to a random encounter with about 8 dudes who all happened to be standing at the same altitude, and wiping them all out with one spell from my mathematician.

Freedom Fighters I have a very clear memory of watching one of my downed troops slipping beneath the icy water before I had a chance to revive him whilst assaulting the final base at night over the frozen river.

Peggle

The first time you clear all the orange pegs is great and very satisfying. Then 'Ode to Joy' starts and you feel even better.

Call of Duty 2
Hill defence (on Veteran)

Bioshock
Opening bathysphere sequence

Alan Wake
The dream type sequence in the second DLC

Crackdown
Scaling the Agency tower for the first time

Mass Effect 2
Shepard's Leap for Glory

Metro 2033
Carrying the kid

Rock Band
Nailing five stars on a Hard song after the drum jam at the end.

Left4Dead
First 4 person escape

Devil May Cry
Galleon level

Half Life 2Ravenholm. RAVENHOLM

Super Mario World
The secret star world

STALKER Shadow of Chernobyl
The escape at the end for the 'best' ending.

Mass Effect 2
Running up the side of the building

Resident Evil 4
Freaking out in the hedge maze with the dogs.

Braid
Dat last level. 

More to come.

The Prison of Hope/Latria in Demon's Souls. It does get old after repeating it a few times, but my first run on this level is one of the best game experiences. The dank, heavy atmosphere filled with cackling, random singing, disembodied voices crying out for help, bell ringing and assorted clanks. Suitably spooked and moving forward slowly as if on tip-toes you encounter a new bell ringing tentacle faced enemy who freezes you with a bolt of magic, gets in close and lifts you by the face with his facial tentacles and kills you instantly.
It can only get worse (more awesome).

I'm surprised that Half Life's intro has yet to get a mention.
Also, choosing your difficulty level in Quake by choosing a portal (right? I didn't dream that did I?)

Both awesome for their time.

Op Flash gave Rossi and I countless moments. One of the best was a custom mission that we made excessively hard by accident. Ended with Rossi fleeing from way too many bad guys, getting chased up the castle tower prefab with me arriving in the nick of time in my UH-60 and plucking him off the top. Mission failed but everyone went home.

Jesus. Maybe it was. So memorable eh.

More later, but to start with...

Metal Gear Solid 3
Great boss fights. It had already been mentioned, but the fight with The Boss is one of the best gaming moments. It's so beautiful and is fun too. Being chased by the Shagohod was absolutely exhilarating. 

I loved the battle The End. I was playing with 5.1 surround sound and playing a tactical sniper battle in the beautiful hills woods and stream, animals around, starts raining... beautiful. 

Bioshock
The intro was good, but it's the mid-game reveal that I loved. Great build up and quite deep. One of my favourite gaming moments.

Final Fantasy 7 Aeris, obviously. A close second would be the flashback in Kalm Town fighting alongside Sephiroth and being awed by his power.

Chrono Cross Swapping bodies with Lynx, it was like the JRPG version of Face/Off and completely unexpected.

MGS3 The entire fight against The End and the neverending ladder, genuinely creasing when Snake Eater hits as you climb.

Shadow of the Colossus The moment when a sense of foreboding replaces pride as you kill another colossi.