This is basically a list of games that either defined, redefined, or perfected their particular genre. It's an excellent compilation.
The games on this list have been subjected to "imitation is the sincerest form of flattery" time and time again, because they showed exactly how games of their respective genres should be played.
Certain games being mentioned in the comments here, I agree with some of them others not so much.
Seeing some calls for Portal to be on the list, I'm inclined to agree with that one. Showed how to fit humour, story, and character into a tight well-designed puzzle game. It's short and sweet, and the last half-hour of the game is one of the best gaming experiences I've had. After being stuck in the small puzzle chambers for much of the game, occasionally breaking out into a "behind the scenes" look at the Aperture facility, I felt so empowered as a player after completing the last chamber. There were no additions to gameplay, just a shift in level design and context. I felt like I had dodged the deathtrap and escaped into the bowels of the facility, beyond GLADoS' reach. After spending 2 hours totally at her mercy I felt like the balance of power had shifted between us. That room where the walls are lined with turrets was a great way of using the Portal Gun offensively, and I felt like it was another major step in shifting the balance of power. Well done Portal for crafting in 2 hours the setup to that glorious payoff, done entirely in level design and voice acting.
On the other hand, I wouldn't say Bioshock deserves a perfect 10. The gameplay isn't exactly revolutionary, nor is it perfection of the genre. But the story is excellent and engaging, and it's one of those games with a genuinely valid message about the way video games work. That message, backed up by serviceable and fun gameplay, makes for what I'd call a 9/10 game. The problem with Bioshock is it goes on a little too long. After I offed Andrew Ryan I felt like there was a massive drop off in my enjoyment of the game. The story seemed to have run out of some of its steam, and the lesser parts of the game started to show themselves a little more. I also felt like the game started throwing splicers at me in bigger and bigger droves, which reduced my ability to admire the surrounding environment.