I first played Baldur's Gate when I was nine. It wasn't until I was about fifteen that I actually saw the ending and was able to move on to Baldur's Gate 2. Now I'm twenty six and I still play the series religiously. I cannot pick a favourite out of the two because they're both so good. I consider 1, 2 and Throne of Bhaal to be one big, long saga...heck, even Siege of Dragonspear! It is impossible for me to play one without the others. No other game has had the same impact on me and no other RPG's have ever lived up to what Baldur's Gate provides. There's so much replayability and satisfaction to be found in these titles. No matter the fight, unless you truly are bad at the game, there is always a strategy to help you prevail. There are countless party combinations for you to try out and so many ways to get through the game.
Ocarina of Time is a game I also played when I was very young, and although it perplexed little me and I ended up playing through the Deku Tree on multiple different save files, it was one that stuck with me due to its colourful nature and wonderful soundtrack. The gameplay was engaging and the world was immersive. I did finally beat the game on the 3DS version and have since gone back to play it on the 64 (still have the same cartridge I first played!) and Gamecube and it does not get old to me. My first foray into The Legend of Zelda and still my favourite.
Half Life 2...hoo boy, this game blew my teenaged mind when it came out. Ragdoll physics! Realistic graphics! Satisfying, punchy gameplay! Atmosphere and immersion! It may look a little dated these days, but as far as I'm concerned, it holds up just damn fine. I still remember how creeped out I felt when I noticed G-Man following my progress. I had played it so many times before that and didn't notice him for ages. Then once I did, I couldn't stop seeing him!
Those are my big three anyway, the games that captured my attention and imagination as a wee lad and left the largest impression. I grew up largely on PC. We didn't get a console until early 2003 and it was a Nintendo 64 with only a limited range of games. My only other console experiences were playing Playstation and Xbox at friends houses and not being very good on a controller.