That's EXACTLY what I love about witcher. Great amount of decisions and none of them being right nor wrong. And that's what I expected of Mass Effect: In the end, there was only 'you die' decision and decision, if you save council

for me; the first half life, when the marine throws the satchel charge into that tunnel...
Bioshock when i realised what an idiot i'd been
TLJ for the best "disappointment" in any game ever
Mafia for me, the entire thing played like a good film and the ending was just :-*

In Final Fantasy IV When Palom and Porom turned themselves into stone to save Cecil and Tellah and Yang.....
classic.

I'll never forget the first time I froze a friend solid in a Duke Nukem 3D multi-player deathmatch, then placed laser trip mines around him, froze him again to keep him there and finally tossed a pipe bomb at his feet. I was sat right next to him in a LAN session and was laughing so hard I could barely breath. I let him defrost, ponder the lasers for a second or two, then I blew him to pieces! He he :-)
I think we spent the whole night trying to get each other the same way ..... sigh .... priceless moments :-)

I must agree with the System Shock 2 statements. When I first played it, I could barely play it due to the fear running cold in my veins! :P
On that note, I still can't play F.E.A.R. alone. Cool breeze + dark room + psychic girl = utter terror. AVP 2 is the same. I still jump every time I hear the hiss of an Alien.
On the positive side, I felt a huge sense of accomplishment and pure joy playing KOTOR. I always play through once as light-side, but the dark-side was so tempting that it truly caused me to question my own morals.

One of the first moments loading up Descent 1 and realizing it was nothing like the Doom or Wolfenstein3d I've played before it. And on the similar note, playing online Co-op with a friend (a feature quite a few games still miss today), destroying a reactor and trying to escape the level. Suddenly one of you gets lost and the other flies back to find them, and lead their sorry misplaced rear back out the mine with seconds left. Exhilarating.

mmm, I dont remember exactly...
recently the most excited moment was when I beat the Wii Fit balance challenge.
and a few years ago, I played Arcanum and get very addicted in it. I played it until 6 AM and when I was ready to bed the game crashed and I did't save. My mind was blank and I never play it any more. (indeed I tried, but the in-game music made me sick)

Half-Life 2: EpTwo.
Leave him alone, you bastards!

The most remarkable moment for my gaming experience was - no doubt - the final battle in Heretic (3rd episode). After a long fight I defeated a wizard on a dragon. At least I thought so. Then I realized that the dragon was dead but the wizard was alive! And angry!
Yeah, defeating him took me 3 weeks - about as long as getting through the whole game to meet him.
I really hpe to see Heretic (full 5 episodes) on GOG!

1. The shock of walls/doors/elevators opening up in the dark in the original Doom to reveal a brightly lit pocket full of monsters. Literally made me gasp!
2. The delight of flying through 3d space in Magic Carpet , hiding far above danger, then swooping down between deadly djinni's and such to fight and scoop up goodies. Wow what freedom!
3. Wow what SUPER freedom in Descent! Playing it online with people and learning to think about movement in 6 degrees of freedom was really a mind bender and made for a hugely satisfying kill when you did it right, and a bewildering death or just getting completely disoriented when you got it wrong. The freedom of movement made dogfights incredibly challenging and exhilirating and fun.
4. The sudden appearance of aliens in dark fields and out of dark spaces in X-COM: UFO Defense. This along with Doom are still the only two games that have ever actually scared me. Pretty remarkable that this was a turn-based game and I still jumped!
5. The ridiculously enjoyable gameplay of Heroes of Might and Magic 1 and 2, in general. I felt happy as a pig in sh*t just doing almost anything in this game. Everything just worked so well.
6. In Crusader: No Remorse, when the hero comes back to his rebel lair after a hairy fight and finds all his pals have been slaughtered while he was gone. There was good music there too, a sad, lonesome twangy guitar which you can't find on the music for this game on the net. Or at least I can't. You get to walk around and observe the devastation.
7. The giddy and sometimes slightly guilty joy of setting people on fire in Crusader: No Remorse. They would run around screaming in flames, and it was really something to see back in the day. Along with the largely destructible environment and the control scheme that allowed rolling and crouching, it made Crusader seem like you could do and see much more in it than in other games.
8. The firing of the Big Bertha in Total Annihilation. Incredible sound effects in that game. Get the fire of one or two of these chasing a Commander(essentially like the King in chess, his death is victory) suddenly revealed by a flyover of planes, some of whom are likely trying to bomb him at the same time, and it is absolutely heart-stopping, thundering, leap out of your chair good gaming.

Hahah, I had to do that a bunch of times! Very very fun. And definitely true as to how amazing the freedom of movement was in Descent. Descent was a fantastic dogfight and coop game both.
And of course it was great fun to sit idly chatting with a friend after completing a successful mission he barely survived and then oh so casually launch a few missiles at him ... ah, sweet sweet betrayal!
This was also the first game I found out how incredible it was to have a joystick. A joystick takes this great game ... well, even much better than great.

I'm an old geezer so I've had my fair share of these impacts and here is a couple of them:
1. Pirates on the C64 - my mom (bless her soft heart) bought me the original but right after I started playing it I lost the map! This turned out to be a blessing in disguise as it led to me and my cousin having a great time drawing one by hand as we explored the Caribbean. Doing that added even more to the sense of adventure that Pirates had already given us.
2. Aliens vs Predator 2 - my girlfriend (wife now, she must be mad) had gone to bed hours ago and left me sitting alone in the middle of the night, in a dark living room with my head set on, playing AvP2. I played as a marine and got to the mission where you're supposed to go in and find a nest and blow it up.. so there you are, walking slowly and carefully along dark corridors with your scanner occasionally picking up faint signals, nervous as hell and constantly using up your flashlights battery. Then you're finally at the nest and you start firing and all hell breaks loose. Your movement scanner goes ballistic as aliens pop up everywhere. All you can do is turn and run but you just know it's too far but you can't give up so you run like crazy while occasionally turning to shoot at the closest aliens, which you can't hardly see because your lights out and you've run out of flares. It's one of the craziest experiences I've ever had in any game and I sweating like a pig after that mission.
Ah, that were the days :-)

Star Control might rank up as my most favorite game ever... I don't know if it had a huge emotional impact at any specific time, but the whole experience was awesome when I was younger.
The cradle/sanitarium/scary place in Thief was by far THE creepiest part of any game I've ever played. I almost didn't want to play through it because I was freaked out.
Most startling moment: Decent Freespace 2, when you go through the 2nd subspace portal in the Nebula and all you hear when you emerge is your wing leader screaming for you to dive as a gargantuan Shivan cruiser is point blank and heading straight for you... After a split second of OMGWTFIMDYING I afterburned the crap out of the way. Well done.
Honorable mentions to Lands of Lore (first two), Planescape.

Oh yes, there was nothing better than starting a new mission, "accidentally" getting hit in the back with a Conc or Merc missile, turning around, and shoving an Earthshaker right in their face. I sincerely hope interplay gets the funds they need to start Descent 4.

Most frightening: System Shock, especially the 3rd level, until you find the light switch, you are completely blind.
Saddest: FF6, when Celes tries to commit suicide. Aeris' death in FF7 comes close too.
Most satisfying: the whole of the dark-side ending in KoToR, especially the way you manipulate the republic fleet into getting destroyed. That was one of the few times cyou ever get to do something clever and manipulative as an evil character in a game.