You know that one game that changed how you perceive games in their entirety.  This is generally a good experience but can can bad experience as well. 

For me this one game compelled me so much that I had to talk about it.  But who was I to talk to?  My younger brothers or parents? Nah.  My friends in middle school?  Sure but not everyone had played it yet or even owned the system and the one's that did didn't have the same passion that I had.  It was 2002, a year after the Dot.com boom/bust ended.  During said peroid (1999-2001) my parents had gone out and gotten us a computer but me and my brothers never used it.  But I remembered that my dad used it to discuss online with other people that had a similar passion of motorcycles.  So I did the same and found myself at the IGN boards (I know, but they didn't suck back then) and indulged myself.  This eventually introduced me to internet culture, gaming culture, comic book culture, and so much more.   Sure something else would have eventually compelled me to discover the interwebs but this was it.  For me this game changed me from a casual gamer to a hardcore gamer. 

And that one game was Metroid Prime

Had I played a Metroid game before?  Nope.  Did I even know what the Metroid series was?  Minus playing Samus in Super Smash Brothers, nope.  There I was in a Gamestop after buying my Gamecube as well as having a copy of both SSB: Melee and SM: Sunshine.  I had enough money to buy one more game and I chose Metroid Prime.

This was one of the first games that I literally played everyday until I beat it.  Then once beaten I see "hard mode unlocked" and continued again in this fashion.  Earlier as a casual gamer I would play when I got bored, and once beaten I was done.  Get all the items?  Nah I already beat the game.  Play hard mode?  But its too hard!  Metroid Prime was just that changing to me, I felt the game wasn't complete until Hard mode with 100% items was done.  Then came speed running......  To this date I own every Metroid game in existance.  The super hard to find Metroid II.  Other M....yes that game... Metroid Prime Pinball! Yes there was a Metroid Pinball...... And the rest as well!

I could go on about how amazing the game is but I'm not here to write a review and I'm sure most people know about it or have played it enough that they do not require me to explain why this game is so amazing.

 It made me want to experience the best that gaming had to offer.  That drive got me to want to talk about it online and introduced me to the game culture.  That drive got me to make a dedicated PC to which I could play games at their best.  That drive changed who I am.

And its been a fun life so far.

So how about you?

mario 64. I could jump everywhere!

Final Fantasy VII, it opened me up to a completely new genre and style of gaming.

In all honestly? Phoenix Wright. This game (the only game that has given me goosebumps, ever) proved me that good ideas can equall a good game. Even if you are not the kind of gamer that plays a game for its story, I think this one is pretty powerful. Oh, and I almost cried at the end of Trials and Tribulations.

When I was a kid, another game that moved me was Fable, actually. I could not believe all the things you could do in a game before playing Fable. Oh, and loved the style. Very..."fablish" :P

Wind friggen Waker.

EDIT:Actually it's a tie between WW and Tales of Symphonia.

Wonderbook.

The Legend of Zelda: Ocarina of Time

Tales of Symphonia was the game that changed a friend of mine forever xD I think he has beaten it 100%, twice.

WW was pretty amazing too.  REMAKE DO US PROUD!

Tales of Symphonia has a special place in my heart cause of playing multiplayer with 3 other buddies.

...and arguing who lloyd should hook up with.

Zelda: Ocarina of Time, the game that brought me to the Nintendo side.