I awed seeing Hyrule Field for the first time in Ocarina of Time. Another game that has awed me is Assassin's Creed 2, since it managed to build on it's predecessor and become one of the best games this generation. The franchise itself is probably the best new franchsie in 10 years

Off the top of my head... Mass Effect and Mass Effect 2

Shadow of the Colossus.

Oblivion

Fallout 3

Baldur's Gate 2

Call of Duty 4

Suikoden 2

Demon's Souls

Uncharted 2

There's no way I'm going to list every game that has wowed me in the 20 years, that I've played video games, so here's just a couple of the most breath-taking ones:

Super Metroid, Metroid Prime, Shadow of the Colossus, Super Mario Bros. 3, Ocarina of Time, The Wind Waker, Chrono Trigger, Resident Evil 4, Bioshock & God of War III.

Super Mario 64

Shadow of the Colossus

Demon's Souls

Terranigma: When I first played it I thought it was great but I didn't expect the depth it offered (somehow I thought going through the five towers in the underworld was pretty much it ). When I first set my foot on the surface I was wowed. Goodness, this game is huge, and now I'm alone going to resurrect life on earth! It was magical seeing the first plants growing; seeing the first animals. There's so much greatness in this game it's painful knowing how unknown the game actually is. How 'bout a VC release SE!?

Zelda: Ocarina of Time: OoT is a long series of "Wow!"-moments but the one I rememer the most is when you get Epona. Escaping Lon Lon Ranch and realising that I have a goddamn horse now was a truly awsome experience. That may be my most favourite moment in a game ever.

Demon's Souls - entering the second area of Tower of Latria. My jaw literary dropped at how gorgeously horrific the level looked.

Silent Hill - The opening level. From the moment Harry leaves his jeep in the search of his daughter until he's killed in a dark alleyway by demonic children only to wake up in a cafe realising it was all a dream. This whole sequence is still stuck in my head since I was kid.

Klonoa: Door of Phantomile - playing it for the first time.

Half-Life 2 - stepping out of the train into city 17. I played this game in 2007 and I was still blown away by the atmosphere.

Assassin's Creed - climbing the dome of the rock mosque.

trying to think of more...

I'm gonna go through my entire history of video gaming.  Here we go.

Dragon's Lair:  (arcade) At the time, when video games were nothing but Space Invaders and Pac-Man, this game came out and was as beautiful as a Saturday morning cartoon.  Of course, it was no more than a video that let you make choices as to which video would play next.  Still, at the time, I couldn't believe my eyes.

Super Mario Bros. 3: (Nes) Super Mario Bros. set one hell of a standard.  Super Mario 2 was very good but very different.  When I got my hands on SMB3, I never looked at games the same again.  The tricks (infinite 1-up traps), the secrets (like the coin ship), the gameplay, the graphics, the scope--my idea of what was possible in a game was blown wide open.

Street Fighter 2: (Arcade) I was in an arcade back in the early 90's and there was a crowd around one of the machines.  It was SF2.  I didn't even play.  I just watched.  It was the most fluid thing I'd ever seen.  People were jumping in using different characters, and pulling off the most amazing moves I'd ever seen.  I've been in love with the game/genre ever since.

Star Fox (Snes):  I'd played games in a 3D perspective--World Runner and F-Zero spring to mind.  When I got my hands on Starfox, I got a glimpse of the future.  I used to just sit there and watch the intro, over and over again.  This was real 3D and it was in my house.

Donkey Kong Country (Snes):  Great gameplay had become a bit of the norm, on the Snes.  Great graphics, too.  But at a time when the competition was pumping out CD peripherals and such, Nintendo and Rare showed what they were all about.  I recieved a VHS tape in the mail.  It was pretty much a Donkey Kong Country commercial.  The graphics were like nothing I'd ever seen before.  The music was beautiful--to the point where I STILL listen to it from time to time.  Once again, Nintendo had taken a new technology, tied it to a solid game, and blew my mind.

Chrono Trigger (Snes):  Maybe there ARE better RPG's than Chrono Trigger.  I refuse to believe that, though.  This was my first foray into the RPG genre and the gold standard by which all others will forever be compared.  It was the story that was on a scope I'd never experienced before.  It was the characters that were all memorable, likeable, and diverse.  I actually felt like I had some investment in the story and relationship with the characters.  It was the gameplay, complete with the first "New Game " that I'm aware of, combined magic techniques, and "Active Time Battle".  It was about time, and it was amazing.

Killer Instinct (Arcade):  Again, it was the graphics.  I never really liked Mortal Kombat.  The controls just didn't do it for me.  Here, we had a game that controlled like Street Fighter, was brutal likeMortal Kombat, and had a presentation like nothing I'd ever seen before.  This game was as much fun to watch as it was too play.

Super Mario 64/Zelda OoT (N64):  How many times do you play a game and know without a shadow of a doubt:  "This is the best game that the world has ever seen."  Twice. That's how many. These were those two times.

Tekken 2 (PS1):  My neighbor had a PS1.  In my head, I just knew that it sucked.  I sat there and watched him play Street Fighter Alpha.  I wasn't impressed.  "Look at the load times!  Look at the shitty characters!", I said.  Then, he tossed in Tekken 2.  I couldn't even hide my amazement.  The CGI cutscenes, the animation, the depth of moves.....it wasn't long before I bought a PS1!!  And when I bought my PS1, I also bought........

Resident Evil (PS1):  The intro didn't impress me.  The graphics were pretty good.  The controls took some getting used to.  But then, I went down that hallway.  ZOMBIE DOG THROUGH THE WINDOW!!  I dropped my controller and screamed like a little bitch in front of my little sister and my cousin.  We laughed.  I killed the dog and talked about how awesome that was.  Two seconds later, it happened again!!  It was the first time a game had actually scared me.  From that point on, I realized that games had the power to immerse you in atmosphere like nothing else, out there.

Metal Gear Solid (PS1):  My brother and I were Solid Snake fans since the Nes.  We even stole Snake's Revenge from our local video game store.  So, when I heard Metal Gear was coming to the PS1, I knew I had to have it.  I bought it on my first date with my eventual fiance (we still talk about that!).  The highlight of that night wasn't a good night kiss or anything.  It was me going home and playing the most cinematic game I'd ever seen.  This was one of the few times I would classify a game as "Absolutely perfect".  I use the term "awesome" to describe everything from shoelaces to pizza dough, these days.  Back when I played Metal Gear Solid on the Playstation, I called the game "Awesome" and I meant it.  I was in awe of how good this game was.

Gears of War (Xbox 360):  I was happy with last gen but it seemed like a better looking version of the previous gen.  This gen seemed like a better looking version of the 6th generation.  For the most part, it was.  I'd played and loved RE4 on the 'Cube and I didn't really plan on buying a console this generation.  This game (combined with the hype building for the Wii) made me change my mind.  Most of what I'd seen of the Xbox 360 left a bad taste in my mouth.  It didn't seem like a generational leap.  When I finally saw GoW running in person, I couldn't deny it.  The next gen was here.

Wii Sports:  (Duh!):  Yeah.  It was a trick.  The Wii didn't track my movements 1:1.  At the time, I didn't know it.  When I got my hands on my Wii (that sounds dirtier than I intended, honestly) I was impressed.  The crude Mii graphics quickly gave way.  When I punched, the character on screen punched.  When I swung the remote like a tennis racket, so did my onscreen avatar.  My imagination filled with what the future might hold.  Of course, much of what I imagined still hasn't been realized.  At the time, I can't deny that I was in awe.

--Off topic:  My kid played Space Channel 5 part 2 on the PS3 (B/C) this morning.  Somebody needs to port this game to Kinect, ASAP!!  Yes, it's pretty gay but then so is pro-wrestling and I like that. 

 I just remember that feeling when I left Midgar and found out the world I had spent 10 hours playing in was just a spec on a huge world map. My jaw dropped, it was just breath taking for my little mind as a kid, no game has wowed me again like that.

 Hell the whole of FFVII was just amazing, simply because it was so much bigger then anything I'd ever played before. 

@forest-spirit - I remember in Terranigma too thinking that after beating the scorpian boss at the end of the 5 towers that was about it for the game. I did it a few hours after getting the game and was beginning to become disappointed - but then it turned out I had barely scratched the surface. It was a pretty amazing game tbh, I really hope we see another RPG be based in the real world too as it could be done so well with todays graphics and whatnot.

Final Fantasy VI

Oblivion

DKC

Vectorman

Ocarina of Time

Super Mario 64

Killer Instinct

Perfect Dark

Warcraft III

Age of Mythology

The Battle for Middle Earth (underrated masterpiece)