I've been gaming for a while.  I've played a lot of games, but I haven't played everything.  Still, there are certain games that you play and when you finish, you think "THIS is why I'm a gamer".  Those moments are few and far between, though.  Still, when you find one, you know you've just been a part of something special.  Here's a list of games that raised the bar for me:

1.  Pac-Man: (addictive gameplay) I don't think that this was the first game I ever played, but it WAS the first game I ever loved.  I'd fall in love many times, over the years, but Pac-Man was the first. 

2.  Super Mario Bros.: (imaginitve new world) I'd played Donkey Kong.  I'd played Mario Bros.  When I got my hands on Super Mario Bros., I knew it was something special.

3.  Super Mario Bros. 3: (gameplay perfection) When I played this game, one thing was certain:  This game was better than everything that came before.  EVERYTHING

4.  Street Fighter 2: (multiplayer phenomenon) It doesn't ignite the same fire that it did in 1991, but at the time Street Fighter 2 wasn't just a video game.  It was a phenomenon.

5.  Star Fox:  (3D graphics) Not the first 3D shooter, but it was MY first 3D shooter.

6.  Super Metroid: (emotional connection)  The gameplay wasn't entirely new (my first Metroid was part 2 on the Gameboy) but when I was finished with this game, all I could do was sit there, looking at Samus and say, "Whoa."

7. Chrono Trigger:  (flawless story) My first RPG happened to be the greatest RPG ever.  Your opinion is wrong.

8.  Super Mario 64/Wave Race:  (next gen greatness)I got both of them at about the same time.  It was like my eyes were opened.  It was the introduction to a new world!  The arrival of the N64 was pretty much the jump off point for everything we take for granted, today.

9. Metal Gear Solid:  (incredible presentation) Another example of a game that made you think "gaming will NEVER get any better than this".

10. The Legend of Zelda: Ocarina of Time: (the greatest game of all time)  and then, about a month later, I thought "gaming will never get any better than THIS".

11.  Metal Gear Solid 3/Resident Evil 4: (perfection) Probably the last time a video game has taken my expectations and totally destroyed them......in a good way.  No game(s) since has given me that feeling of "Holy shit, this is awesome" like these two did.

So, that's my short list of the big ones.  The games that blew away everything that came before them.  There are of course better games available now, but the games I listed are the ones that set the new standard, and not just refined them.  These games were the ones that gave me my defining moments.  So, which games gave you that feeling?

The Legend of Zelda Spirit Tracks. I thought it won't surprass the fun of Zelda games on consoles. Now it did and is my 2nd favorite Zelda

1. Super Mario Rpg: Legend of 7 stars... awesome graphics, great music, perfect rpg gameplay
2. Chrono Trigger: The most epic story ever, I couldn't believe and I played it like 9 times

I actually had no interest in this game, whatsoever.  Now, based on your recommendation, I may have to go out and buy it. 

I'm trying hard not to be that guy that responds to every post in a thread, but: Mario RPG!  Fuck yes!!  I loved that game so much, at the time.  I need to get that on the Virtual Console, today! 

d21lewis, is this gen lacking 'defining' games to you?..my list looks a lot like yours :)...except to me, MGS3 isn´t there (though I still think it´s a very good game) and Resident Evil 1 and 2, Symphony of the Night are :)

For it's time, Space Invaders was phenomenal. I also was a big fan of Dragon's Lair.

I am also showing my age.

- Final Fantasy X: Incredible sense of adventure, throwing you from one place to another, and from one situation to the other very quickly. The characters are vastly enjoyable, as is the awesome story.

- Chrono Trigger: No other game allows you to see such a spectacle: Watch your actions in the past affect the future, see the glory and vanity of the Empire of Zeal, explore a wide variety of locations, from prehistoric times to the far future, all with an AWESOME cast. See pure evil and kick its' ass. Great game.

- Digital Devil Saga 2: Experience life after an apocalypse. I'll keep this short. Awesome cast, awesome villains, awesome music, awesome graphics, awesome battles. Just play it.

- Metal Gear Solid 3: Channels an excellent atmosphere. No other game does the cold war better, and no other game does the jungle so well, too. Some very epic moments in it, too.

- Tales of Symphonia: Now, I know I'm gonna get flacked for this, but this game is very immersive, with such a great battle system and nonexistent load times. I played it for 5 hous+ every time, because it never got tedious.

Hopefully, by the time this gen is done, my opinion will change.  Even with innovative hardware like the Wii and the DS, it feels like most of the games this gen have opted to "play it safe" rather than push the envelope.  The closest thing to a defining game that I can think of is Braid on the PSN/XBLA.  I'm enjoying the games, that's true, but it seems like we're just getting more refined games instead of revolutionary software.

Half-Life 2: Possibly the best way to tell an amazing story in a FPS. The atmosphere was one of the deepest/best i've ever seen. Combined with the outstanding KI and physicsengine this game felt so real.