Doom
Jazz Jackrabbit
Commander Keen: Keen Dreams
Wacky Wheels
Blood

The Best one was Doom, I learned the first episode by heart that by the age of 10 I was doing it on the "Ultra Violence" difficulty....so much for that 18+ age rating :D

Star Wars: Dark Forces 2: Jedi Knight - Good blocky polygonal times.
Destruction Derby - I must have played that demo to death until we got the game one Christmas. Arena-car-smashing mayhem just didn't get old. ;)
Full Throttle - I didn't actually get to play the full game until 2004 I believe. Thanks to ScummVM and a buckaroo for an old Mac version.
Magic Carpet 2 - I never got to play the full sequel until it appeared on GOG.
Halloween Harry - I never did play the full version. I ought to track down a freeware copy one of these days.
Duke Nukem 3D - I didn't get to play the full game until the early 2000's, but the kicking ass was always fantastic.
Shadow Warrior - Similar story to Duke 3D.
Screamer - Played this arcade racer over and over again back when. Unfortunately, it's not nearly as fun today, and that constant popup kind of kills it for me.
Terminal Velocity - Fly around and blow shit up. What's not to like?
Nitemare 3D - I had way more fun with it than I probably should have. It wasn't even that great back then, but for a kid with few options I took this mediocre shooter's shareware episode and reveled in it.

I'm sure there are plenty of others I can't think of right now.

Trespasser. Which had a custom level that showcased the type of physics based puzzles they wanted more of in the game from back in 1998 I think. You might be able to still get it from the usual download sites.

Demos were basically all I had in early 2000s for my Pentium 2 PC. Some of them I played for weeks or even months even though they had only one level, especially RTS games:

Cossacks
Worms Armageddon
Sudden Strike
Earth 2150
Warlords Battlecry
Machines
Desperados
Robin Hood The Legend Of Sherwood
Tony Hawk Pro Skater 2 and 3
Max Payne
Lego Racers
Not sure if shareware counts, but I played a ton of Soldat against bots.

Demos? I don't remember demos. I remember shareware.

Like a good chunk of Exile 2: The Crystal Soul. That was awesome back in the day.

Well, even though the demo is technically nothing special I must mention the MDK demo here. I was so blown away by it as a kid, I kept playing it over and over, must have finished it several dozens of times. It was actually a very short demo that could be, I think, finished in less than five minutes. It provided just the right amount of content, though, to really make the game shine. The same goes for Outlaws, I guess (which, I think, I actually got from the same CD as the MDK demo). They chose by far the game's best level for it (the second one) which is the ultimate Western experience. And the nature of the game and this specific level in particular meant that I could play it dozens of times without ever getting bored, always trying new tactics, new routes and searching for secret areas.

Then there's of course Half-Life: Uplink which stands out due the amount and quality of original content created solely for a demo. I got so hooked on Uplink, I was kinda devastated when it occurred to me that it's not a portion of the full game and I will never get a solution to its cliffhanger ending (damn, Valve's been doing unsolved cliffhangers all along!).

Including classic shareware I don't even know where to start as a lot of my DOS gaming experience was entirely based on shareware games. I guess the shareware version that stood out most must have been Shadow Warrior, though, thanks to providing an entire mini-campaign that felt like a complete (albeit short) game. And those were some really damn fine levels, frankly a lot more memorable, iconic even, than any level from the full version's main campaign.

For me, the top was Battlefield 1942. That introduction to 32- and 64-player online FPS on the Wake Island map was a revelation.

I played Tomb Raider demo a lot back then. That was a very impressive game

And also FF8 demo. First time seeing leviathan summon is mesmerizing

I don't get why more online demos aren't available today, you'd think it would increase sales but maybe not

I did, and still do, find games that had original content as demos very interesting.

A few have been mentioned already in here, like Jedi Knight 2: Jedi Outcast, Desperados: Wanted Dead or Alive and Half-Life: Uplink. StarCraft’s demo was entirely original too, and kinda functions as a prequel to the game campaigns. Warcraft III had some original levels too in the demo, expanding the tutorial to tell more of Thrall’s journey to Kalimdor.

That said, I played a lot of demos and shareware in my younger days. Exile: Escape from the pit (Original art set) was one of the real highlights, giving a MASSIVE shareware to explore (Also, bonus points for the shareware demon, whom I still reference every now and then.)

Ofcourse, I played a lot of the classics too, like Doom and a bunch of Apogee shareware titles.

Diablo 2 was also one of the demos I played a lot, but that demo isn’t very remarkable.

There are a few others that stand out, like Army Men: Toys in Space. For some reason, the demo consists of the penultimate level of the game. You’d think they’d pick something much earlier, but no. Same goes for Army Men: Air Tactics, in that is features a very late level.

You made me bump my DEMOS which has full games thread.

It could be interesting to collect game demos with levels that aren't in the final game, if nigh on impossible. I recall Fallout: Tactics had two such levels that are available on Youtube. The first was a heavy alteration of mission 1, and the second is an unreleased map that serves as a prelude to taking down the main raider base in mission 3. The mission briefings were all voiced by Todd Susman who played the imposing Paladin in a cinematic. He does a good job, but there's just no comparison to who they went with in the full game - R. Lee Ermey in Full Metal Jacket mode. Between him and Ron Perlman doing his classic intro sequence, this game was an auditory orgy.

It could indeed be interesting, maybe even as GOG goodies along with the games. I can't remember, but did the demo for Heroes of Might and Magic 3 have an exclusive level? I rember the demo being really hard, but I was rather young back then.

I bought Doom, Hexen, Ascendancy, Quake, Quarantine, Syndicate, Syndicate Wars, X-Com, Terra Nova, Hitman: Codename 47, Blake Stone: Aliens of Gold, System Shock, Heroes of Might & Magic, and Crusader: No Remorse solely on the strength of the demo/sharewares. I also played hours and hours of the sharewares of Wolfenstein 3D and Duke 3D.

Funnily enough the demo for that put me off the game as it didn't run that great on my PC and seemed it wasn't as good as similar games of that time like Operation Flashpoint.