1. Metal Gear Solid 3 - Best game story I have ever witnessed, best game of the series, and an all round great stealth game.
2. Uncharted 2 - The way the story was put across was the most cinematic of any game I have ever played. Cutscenes were important, and instead of "oh look at this pre-rendered guy running away from that big thing" you control him. The gameplay is a nice mix of Tomb Raider and Gears of War that works so well.
3. Kingdom Hearts II - I absolutely loved this game. It improved on all the combat in the original, the story was decent enough, exploring all of the Disney worlds was so fun and they all were so beautifully colourful. JRPG set in Disney worlds = awesome!
4. Half-Life 2 - Really improved in-game physics to the standard they're at today, set new graphical standards, totally original weapon set, decent story told in a way that completely strayed away from pre-rendered cutscenes. It also was the 1st implementation of Valve's source engine which made a massive impact on the modding community.
5. Guitar Hero - The original Guitar Hero. Back when it was a new concept, when we were all noobs, and the soundtracks weren't scraping the barrel. This brought rhythm based games out of the shadows and into the mainstream. I also can say from observing a lot of gamers I know, that it has influenced multiple gamers to learn a real instrument, assuming more gamers than just my friends share similar mindframes.
These aren't in any particular order. These are my top 5.
My 6 to 10 are:
Halo - Improved console FPS controls
GTA 3 - Improved sandbox gaming
Forza 3 - Greatest driving Sim to date
Timeplitters 2/3 - Best local multiplayer FPS games I know of
Tekken 4 - The only true 3D game of the series in my opinion, with the most balanced character set, and great environments