What do you think are the best, coolest, most memorable, most iconic, etc tech demos in gaming, to you?

Maybe they set lofty promises reality didn't quite live up to, maybe they offered mind-blowing glimpses into an upcoming generation, maybe they were just badass, whatever the reason, which stand out in your mind?

Dark Sector tech demo 2005. First "next gen" game showed for the 7th gen. Of course the final game Dark Sector was very different and what the original Dark Sector was supposed to be eventually became Warframe.

The worst tech demo ever because it almost ruined people on Wind Waker. The best 3D Zelda at that point.

Unreal 5 Engine demo (Epic) - When I look at games today most are always constrained by polygon budgets and you can see simple geometry dotted around to make up for it, which is a bit gamey. UE5 is the first demo with new tech with Nanite (mesh shaders) which absolutely almost destroys that concept and blows it right open - superfine detail and insane geometry is now achievable with the tech today and thats exciting stuff for me. I mean I haven't seen that level of detail in one interactive scene in any game or demo before. Crazy stuff, can't wait.

Kara (Quantic Dream) - Okay, so the game (Detroit: Become Human) wasn't that great or lived up my expectations but the tech at the time was super impressive  at the time with the facial expressions and rigging on Kara on PS3. And the premise of a cyberpunk game, mixed in with androids was pretty exciting concept indeed. Shame, the game was so disappointing :(

It's not quite there yet, but getting closer.

Biased answer but; the 2011 Zelda WiiU demo. It probably hurt Skyward Sword afterwards because it didn’t look as cool, and made TPHD a bit of a disappointment because it was also way inferior years later.

Serious Sam

Most people don't know about it anymore these days, but Serious Sam was developed as a tech demo for Croteam's Serious Engine. Hence why you also have a demo level which showcases all the tech in the game. Even the Karnak temple demo was designed to showcase as much of the capabilities of the Serious Engine, especially during the intro, which shows you most of the level (minus most of it's secrets and later spawning-in ennemies).

https://www.youtube.com/watch?v=9B3NHaiScKo

The Demo was so well accepted by gamers that instead of selling the engine, they ended up selling a ton of games

Edit: Also memorable: NVidia's Dusk and Dawn demos. Most memorable because they ran better on Radeon GPUs than on NVidia's own GeForce FX...

https://www.youtube.com/watch?v=bw2aEklugFQ

https://www.youtube.com/watch?v=um8pAamNn_o

NVidia remade that last one later on for more modern GPUs btw:

https://www.youtube.com/watch?v=bI1_quVr_3w

That 2011 Zelda E3 demo kind of stings in hindsight because it 1. really made the Wii U look much more powerful than it actually was, and 2. before that, the actual Zelda games always looked much better than the early Zelda tech demos which showed off the new systems. Ocarina of Time looked so much better than the tech demos for Zelda 64, and Twilight Princess looked so much better than the 2000 Spaceworld Zelda. Breath of the Wild looked great and going open world meant the system's power was going in different places than the demo, which was in a single room, but this was the first time a Nintendo graphics demo oversold the power of the system instead of underselling it, and we never really saw an actual game that reached that level of fidelity on the system.

The Order: 1886. Oh wait...

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I guess this left a bitter taste since it has not been mentioned yet. It did look impressive.