For me it was definitely Final Fantasy IV (or Final Fantasy II as we called it in my day).

I was 11 years old or thereabouts and was spending a few weeks at my grandmother's house during summer vacation. She lives in a little hole in the wall town with absolutely fuck all to do, so I naturally spent a lot of time with my SNES and the games I rented from a Take Two video store that smelled suspiciously like a barn.

I'd get a new game every couple of days or so and quickly burned through the likes of Young Merlin, Super Black Bass, and the so, so awesome Lost Vikings. Each time I'd return an old game and go to pick out a new one, I'd find myself mesmerized by this simple yet enticing red box that said only "Final Fantasy II". The "T" was a sword, and it was so fucking classy, baby! Way too classy for a store that smelled like a cow's ass. It was kind of like that scene in the Neverending Story where the kid sees the book and can feel it calling to him. This game was my destiny. But while I'd played some RPGs here and there like the Ultima series (or at least the dumbed down NES versions), I was once touched where my bathing suit goes by a Wizardry game and had kind of shied away from them ever since. So I'd always put that sexy red box back and check out something else instead: ClayFighter one day, Super Double Dragon the next, and so on.

Then one day I rented Romance of the Three Kingdoms. Now, none of these games came with instruction manuals, and if Wizardry was like being felt up by a friendly uncle, ROTK was like being gang raped in a porta potty by his golfing buddies. Figuring I had nothing left to lose, my aching brain and I promptly took it back to the store and exchanged it for "Final Fantasy II".

Mother of god. This game had EVERYTHING! Cool looking monsters and an engaging story and a musical score that blew my fucking miiiiiiiiiiiiind. For as long as I can remember I have always played video games. Even when I was too little to know what I was doing, I would fire up my dad's Atari 2600 and crank away at the joystick to try and make things happen. But this was the first game I'd ever really played compulsively because I just had to find out what happened next.

I'd never really considered games to be a medium for storytelling before. They were just things I used to kill time because I was a bored kid. Their stories never really seemed to go beyond "save the princess", and that was always good enough for me. Final Fantasy II changed that forever. Climbing Mount Ordeals and becoming a paladin was an epiphany for me: it was the first time I really ever felt for a video game character. More than that, it was the first time I ever felt that I was a video game character. Cecil's journey was my journey, and to this day I can still remember making it inside the Giant of Babil and hearing the fucking EPICCCCCCCCCC "Within the Giant" (FF4 marked an epiphany for me in terms of video game music as well) and just putting down the controller and walking around the room, marveling at how far I'd come.

What a legendary game. (Edit: And it's nice to see I'm not the only one who thinks so!)

Pong

Drops mic, walks away.

I've played every single Ace Attorney, 'xcept for Investigation 2 xD. I need a 3DS ASAP, as Ace Attorney 5 and Professor Layton Vs Phoenix Wright are in there :P

You should try Ghost Trick. Another Shu Takumi masterpiece.

I have been meaning to do that, but the game is too expensive at the store where I usually buy games. That store is also the only store where I have seen that game too. When it drops in price I will buy it.

Can't wait for AA5 and PW vs PL, both of them are among my most anticipated games for the 3DS and in general.

hmm, after seeing some games mentioned by some, I would like to add:

Pokemon - Will forever be 'the one'. I never knew games could be like this.
God of War - Definitely changed the way I look at games.

The Secret of Monkey Island

I always loved gaming, but i was never a good gamer reaction wise, i never finished a game without using some kind of cheat or looking up solutions. There i said it!

But suddenly there was this game that wouldn't kill me, it allowed me to take my time and figure everything out on my own and it was the fist game i beat without any help.

I almost exclusively played point and click adventures for years after that, in turn with certain turn based RPG's

...the fact that i am playing main stream games again today too is because they became so goddamed easy in this gen

For me, there are two. That's as best as I can narrow it down.

Final Fantasy VI -- Starting this game up on my SNES was the most important gaming moment of my life, without doubt. Prior to that, in all honesty, gaming was nothing more than a way to kill some time. I'd enjoyed my NES a lot, but the SNES just felt like more of the same. Platformers, beat'em ups, rinse and repeat. Other than Zelda, I was drifting away from gaming and getting into reading more than ever before. Then I happened to see FFVI at a used game store that I was only in because of my friend. I bought it on impulse and it blew me away. Depth in video-games, a fantasy world with multiple, well-developed characters, with love and betrayal and politics and an actual story. This is the game that made gaming my hobby, that got me online looking for other RPGs, that made me stumble across anime and Asian films and television. This is the game that directly led me to the PS1 because of FFVII, that prompted me to pick up Suikoden and Final Fantasy Tactics.

I owe a lot to that little gray cartridge with a Moogle on the front.

ICO -- I compare this game to Serial Experiments Lain a lot. Both of them kicked down walls in my head and showed me the vast potential their respective mediums actually encompass. I bought ICO when it came out, I don't even remember why. There was no internet hype back then, no expectations, so when it hit me, it came totally out of left field. I remember clearly the bridge sequence and how I just sat there, stunned, with the controller loose in my hands. It wasn't just a videogame, it wasn't just something I was playing, it was a personal experience where I absolutely and completely hated the antagonist, where I wasn't just trying to finish a game, I was trying to save a tragic young girl from a fate worse than death. I still hold it up, unconsciously, to other games, to see if they've been able to live up to something so simple, and yet so beautiful. For me, ICO kind of changed everything relative to gaming, most of all myself.

When i was young my dad was going abroad to asia and asked me if i wanted any games on GBC (they were apparently much cheaper there) i asked for a dragonball z game (i had one on SNES and didnt know if there was one on gameboy but my brothers powerranger games were annoying me.

Anyways when he got back he brought DragonWarrior 3, at first i was disappointing because it wasn't exactly what i asked for, then i tried it, and i've probably spent more time on that game than any other game in my life, the story was absolutely amazing and the plot twist towards the end was EPIC. I havnt had the chance to play dragon quest 7 yet but so far dragon warrior (quest) 3 will always have that special place in my heart.

I agree with the OP. After playing Metroid Prime, I knew that I would be a gamer 'till the day I die.