The Secret of Monkey Island I & II.
I can't see it being topped even with the renaissance that the genre has had in recent years as the circumstances surrounding my first time playing them can never be replicated.
I was a young teenager and I'd been lucky enough to receive an Amiga 1200 for Christmas. I knew I was getting it, and I'd also been working weekends in a local corner shop doing chores, sweeping up etc to earn some money to buy games for my new computer. I bought the MI games as a double pack (remember when you used to buy games in huge boxes containing 10 or more titles for the same price as a full release?) and from the very first moment the Lucasarts logo appeared I felt like it was going to be special.
The theme music is simply ingrained on my psyche, and still fills me with a sense of longing for travel and adventure. The graphics looked amazing especially when the portraits of the Guybrush and Elaine were shown. The cover art was beautiful
But really it was the humour that bit me. Even on the very first screen I was giggling with the classic 'I'm over here' line to the blind lookout. On the 2nd screen you're looking out to sea and your curser hovers over the moon. I wonder.... you think as you click 'walk to' and then 'the moon'... a moments pause then Guybrush breaks the 4th wall and tells you what he thinks of that idea. I'd never played a game like this, it was funny, it had great characters and I the puzzles were such that even I could manage it. The second game was played immediately after it and simply expanded all that I loved about the first. There was so many little Lucasarts in-jokes and easter eggs crammed in there such as visiting a shop in MI2 and seeing things like Indy's Whip or Star Wars quotes as the silly answers. As a kid who'd grown up on Star Wars and Indiana Jones it felt like the games had been crafted just for me.
However, the other reason those first playthroughs stay with me is because it because a real social thing with my friends. They'd already owned Amiga's and so it became a round-robin experience of heading to a different house each day as we tried to solve a tricky puzzle and each took a turn saving onto our savings discs so we could continue at home. If one of us solved it, we'd go back to the last bit we'd all seen so we could show off the solution and no-one missed any of the story.
That's the crux of it, the games aren't just the games. They're wound up in childhood memories of after school clubs, having tea at friends houses or late night sleepovers with gaming and then Match of the Day. It was even Monkey Island that forced me into buying my first PC as with release of The Curse of Monkey Island and its drop dead gorgeous animation on CD-ROM it was also the end of my Amiga. Only this time I was working full time in my first job and that PC was the first system I ever bought with purely my own money.
Sure there have been better games, sure there have been huge marathon sessions and I distinctly remember how excited I was then Mass Effect was released and also playing Dragon Age for the first time being very skeptical thanks to the very odd way it was advertised and slowly realising that I was hooked by its world. But nothing beats childhood nostalgia, especially when you can play it again 20 add years later and still be tickled by the writing and get goosebumps from the music.