Sonic and Rocket Knight Adventures are the two most played games from my childhood.

Sonic the hedgehog, crash bandicoot, and spyro

and the Simpsons arcade game, on an actual cabinet.

I think it would be "Cauldron." (ZX Spectrum), a mid 80's game.

Legend of Zelda: A Link to the Past.

Also Super Metroid, Mario Bros. 3, Secret of Mana, Chrono Trigger, Tetris and Pitfall.

Esotera:
Civilization III and Shining Force. Both of which I'm currently replaying. Pokemon and Rome Total War also make it up there, although they're a bit more recent.

I'm still playing Civilization III. :-) Timeless and addictive games like Civ III appear in the video games landscape once in a decade I think.

Christ that's a hard one. I would have to say Pokemon Yellow, never have I put more hours into anything else before or since getting that game. Not even the newer Pokemon games.

Pokemon Red. It has to be Red, Blue was such a bad game, Red was so much better!

Well, umm, this is a bit different from most that I see here, but Command & Conquer, and Command & Conquer Red Alert.

I still love that *squish* sound you hear when running a soldier over!

Probably either Legend of Zelda: A Link to the Past (SNES), Super Mario RPG: Legend of the Seven Stars, or Majora's Mask. I remember that Link to the Past was the first game I actually beat, and I was so young at the time the game actually helped teach me to read. Super Mario RPG was just awesome, and Majora's Mask is probably one of my top 10 games of all time (Spoiler: Ocarina of Time isn't in the list).

Godzilla Monster of Monsters was it for me.
2D side scrolling running around punching and kicking other monsters. It even had RPG elements in it. Not very well implemented mind you but they were still there and I loved it for them. I'd always just crash Mothera and just play with Godzilla.

Legend of Zelda: Majora's Mask because it was and still is one of my favourite games. It also reflected my personality well be being extremely creepy.

Edit: Also any (and every) early pokemon game, since I was a kid in the 90s.

Those light gun hunting games you sometimes find in arcades, usually in bars and places like that. I just loved those as a little kid for some reason. In fact, thinking on it now, the love of fictional hunting those games instilled in me might be part of why I love sniping so much in the games I play now. I even still like to take the time to hunt the occasional wild animal in any games where I can, like Red Dead Redemption and Skyrim.

Legacy of Kain : Soul Reaver 1 and 2. Now that's one series I will never forget.

I think the first time I was ever seriously hooked with a game was Myst. Close contenders would be Super Mario World and Killer Instinct.

I destroyed three controllers just because Saberwulf is extremely hard on the hardware, on the long run.

Fappy:

Shoggoth2588:
The Super Nintendo. Cop out you say? "BULLSHIT" I reply! The Super Nintendo is the physical incarnation of my childhood...but if I must chose only one game I guess it would have to be this one:

This is the best game of all time but only because it was able to dethrone the Teenage Mutant Ninja Turtles in the arcade. Konami: We used to be Gods!

FUUUUCK YES!

I love this game. Probably beat it like 100 times.

OT: Honestly there isn't one game I can choose. That's just not possible. If there was though, it would be a Mario or Sonic title.

Damn, i LOVE this game and this cover. Good ol`coop days throwing foot clan soldiers towards the tv front. This is one of my later favorites.

My childhood belongs to LucasArts (especially MonkeyIsland 1&2).

Growing up with my SNES, it would either have to be Zelda: A Link to the Past or Super Mario World. I've played and beaten both of them countless times. I still play them at least once a year

Half Life. Probably explains why I'm so screwed up...

(Or maybe Strife)

The Might and Magic games were a big part of my childhood. My mom and both of my sisters played those games, so I have a lot of fond memories of playing Might and Magic or watching my family play. My sisters and I would even play as a group, with one person on the mouse, one on the arrow keys, and one doing all the other button-mashing. Good times.

Golden Sun 1 and 2. So much Golden Sun.
The only games where I explored and did EVERYTHING I possibly could, including beating Dullahan (who was a bastard).

Sonic Adventure 2 Battle. I freaking loved that game and played it all the time. I spent hours trying to complete all the challenges and in the chao garden. I still have it around and occasionally pop it into the Wii to play.

tekken 2 (man i miss that game) i was badass with law (still am but now feng wei is my fav)

I'd probably say Crash Bandicoot. Not a single specific game mind, the series as a whole.

Terminate421:

I want them to remake this on the 360.....with XBL matchmaking

You are not alone, my friend. I have such fond memories of that game.

As for the game that defined MY childhood, I'd have to say Half-Life 2. Before that, I was a fairly casual gamer. I played simple, non-violent games, and I was happy with that.

Then one day, my father decided to buy a copy of the game. I distinctly remember the art on the back of the box... It had nothing to say about the story or gameplay, and only had a few images, none of which told me anything about the game. I was worried because it was rated M, and my parents had banned M-rated games (a decision I fully supported at the time).

Dad decided to let me play, under the conditions that he would supervise the affair and I would stop if I got too freaked out. The first time I started the game was one of the most memorable points in my life... I can still remember seeing the G-man and city 17 for the first time. Every dead Combine soldier shocked and exhilarated me in a way I'd never known before. Unfortunately, I got stuck on the part in "Route Canal" where you have to cross on top of the train (Wish I'd know that back then... oh well). Also, Mom found out about the game and ordered me to stop playing.

That wasn't the end, though. Even though I had stopped playing, the events I'd witnessed stayed with me. I wanted to return to City 17, and finish what I'd started. Years later, I did. My parents allowed me to go back, and that's exactly what I did. I played that game from start to finish, and loved every second.

Half-Life 2 not only showed me what gaming was, it showed me what it could be. I haven't seen another game that matches the masterful mix of story, graphics, and gameplay I found in Gordon Freemans' second outing (other than Episodes One and Two; those were great). Sure, Bioshock came damn close, but it didn't have quite enough variety to outmatch the One Free Man.

So, to make a long story short, Half-Life 2 is THE game that defined my childhood.
Oh, and Valve? I love you guys, I love the steam service, and I love all the games you've made since Episode Two came out, but you need to get off your asses and make Episode Three, or Half-Life 3, or whatever the hell you're going to call it. Your fans have been asking for it for YEARS.

TL;DR: Half-Life 2 is an awesome game, and it made me the geek I am today. Also, Valve needs to make Episode 3.

The original Half-Life. When I was three, my dad would let me run around on empty multiplayer maps. When I was five, I finally moved on to singleplayer. And thus, a gamer was born.

Xenogears held a massive thick shadow over my childhood. Went through that game like four times before finally beating it.

I know I keep talking about it, but Total Annihilation :D

Gameplay, although this is normal speed, I normally had the speed up to +10. Furthermore, this is a mod.

Super Mario Bros.It was the first game I owned on my first console and I absolutely loved it and played it to death.I still break it out from time to time for a bash

1 game? really? But... damn it. Well, it would either be Spyro 2: Gateway to Glimmar or Ratchet & Clank 2: Locked and Loaded. Probably the former; the latter was more of my tween years.

Edit: Actually no, Sonic Adventure 2 Battle. The game that got me my first real friend, whom is still my closest friend today.

Turrican 2 for the Amiga. I had the demo on a coverdisk- I was very new to gaming and Turrican was quite tough, so it took me many attempts to complete it. Once I did, I explored every nook and cranny of the single level and played it till the disk wore out. 3.5" floppy. Took about a minute to load. Frustrating for what was, by the time I had the level down pat, a five minute demo game, but I learned platform/shooter patience watching my mates wait for Robocop to load on the Spectrum.

-Nick

Playing doctor behind the sand pit, of course. For you guys it was actual video games? I mean, I like 'em, I LOVE 'em, but that's kinda sad.

liquidsolid:
Spyro the Dragon was the defining game of my childhood. An adventure game with eclectic levels made me the gamer I am today. It also indirectly led to me discovering Sly Cooper when I was a little older that got me into my favorite type of game: Stealth.

Kingdom Hearts was also a big one and it got me into Final Fantasy and really made story one of the most important elements I look for in a game.

All the same for me, except stealth isn't necessarily my favorite genre.

Age 6-10 -- Spyro the Dragon, leading to all others
Age 10-13 -- Kingdom Hearts, leading eventually to FFX

That's my life, right there.

EDIT: Oh my god, how did I forget about pokemon.

Age 6-near present -- Pokemon.