When Metal Gear Solid launched on the PSONE I bought it the first day. Mind you I was just 14 when the game launched and I had like no money but I wasn't going to be denied. 

So as you can see, I had huge expectations for this game and bam it exceeded every single one of them. 

The story was brilliant, a rarity for an action game. The graphics were top notch and unlike most PSONE games still look decent today. The gameplay was amazing as well. This game just did and had everything.

The boss battles were for me the key. Sniper Wolf was great especially considering how good it was pre fps boom.

But nothing could compare to the battle with Psychomantis the first time he reads you controller is the biggest OMGFTW moment of all time. Having the game reverse my controls and force me to change controller slots made for the most memorable battle of all time for me. 

Final Fantasy VII was my first PSONE OMGFTW game and still is my favorite game of all time but that game does not hold a candle to the original Metal Gear Solid.

What game had that kind of impact on you?

OMGWTFBBQ was exactly my reaction when Link picked up the master sword in Zelda: Ocarina of Time

Psycho Mantis was another one, I first played it on the GC so I was shocked when he mentioned SSBMelee, Sunshine, Eternal Darkness and Wind Waker.

Oh and what happens during the credits of DKC, that's just cheap.

I was gonna put something mean here but I decided not too after I typed it so I deleted it.

As for me, Ocarina of Time. More recently, Brutal Legend.

Super Mario Bros

It was amazing back in the day.

Was it the music in Brutal Legend that did it for you? Because I though the gameplay was pretty average.

I am not a Microsoft fan but Halo blew me away when I played it. It was omgftw.

It was indeed the music. And the atmosphere. But the gameplay seems like a console RTS for people new to the genre.

Shadow of The Colossus still gives me the sense of adventure i always loved,the feel of burning and the sting of pleasure  as if you can do everything with your will,  i'm not going to say its "Epic" because it goes beyond that and into something so wonderful , so magical , yet so real.

Ico makes me wonder about lots of things ,it makes me feel everything is alright ,like as if i am (ico) and my best friend (yurda) are alone in a far away place and we're just born in it, with the sense of isolation and peace far away from the evil outside world, its very relaxing yet mesmerizing, yet likewise so real

looks like you were a bit late for the party , the gamecube version was pretty weird compared to the original , it had too many overly climactic action scenes , like the part were snake rides evades by backflipping and standing on it .... wtf?!!??!!? ,it totally ruined the point of the first, being that you are an underpowered soldier against a whole military surviving by being stealthy, on Gamecube hes like Duke Nukem Ninja Elite Super Soldier

Edit:

like how they ruined this scene for example

i can't count the things that were painfully wrong with the gamecube version , like when that guy saying "its .... a ghost" on the original version he was talking to you as his last words while barely able to walk, on the gamecube hes obviously lifted in the air by a freaking sword is that what you say when your impaled? , you wonder what the other guy is? " oh its a ghost".... it almost laughable how it was ruined

Lara Croft and the Guardian of Light. Because I could shoot my brother with a grappling hook and annoy him. or little big planet where I could slap him.

but uhh seriously.
It was mario 64... Though FF8 comes close, as well as deus ex. I wanted to put ocarina of time... but I just liked it better than mario 64... I wasn't really surprised by it.

Mario 64, because I was like ZOMG it's mario and I can 3D squash a goomba. And grab bowser by his ass/tail. And there was something that just made me all excited seeing mario in 3D.