Gotta agree with Araknyd and nemesiswontdie, MGS1 is one the few games that has been firmly engraved in my mind and soul as one of the most captivating games I have ever played. And I absolutely love the original PS1 version far more than the remake for Gamecube. The atmosphere, the story, the voice-acting, the action, the stealth, the suspense, the drama... I mean, its got it all and its untouchable. Such a timeless classic. Great, great memories.
Bioshock is another game that needs mention. I hold that game very near and dear to me. The first time playing it was absolutely stunning and gripping. Everything about it was so perfectly crafted. Same as I said for MGS1, it has everything from the compelling story, insane atmosphere to the amazing voice-acting, action, suspense, thills, etc, that is just top-notch in every way. Truly mesmerizing.
I loved Bioshock 2 as well, but it isn't as classic as the first. And I am so damn stoked for Bioshock Infinite.