Deus Ex - So far, the only game in the serie, RIGHT?!?!?! and it is great. The story is just great and I love the RPG elements are just the way I want them.
TimeSplitters 2 - For me, this is the best FPS I ever played. I have spend so many hours in this game alone and with friends that it makes me go nostalgic just thinking about it. The best part of it is that there is just one focus, fun. And that is something devs today have forgotten, save for The Behemoth.
Total War : Medieval 2 - I prefer roundbased strategy, and this is THE roundbased strategy game. I love this game right trough, managing your empire have never been as fun.
Prince Of Persia : Sands Of Time My first PS2 game, and one of the best. Whenever I get angry at Ubisoft for fucking shti up I think of this game and it's perfect platforming and storytelling and it makes me love them again.
Dragon Age : Origins I love West RPGs and tabletop RPGs and DA:O offers the best of both. What I loved about this game was it's characters, and the excellent story. It made me feel things, and not just anger like it Halo 3 after getting sniped by that 5 star general for the 65481485485 time. It made me laugh, it made me suprised and it made me angry and sad when a certain someone that I may or may not have a crush on gave me a ultimatum. BioWare, I can do nothing but smile.
The Elder Scrolls : Oblivion - I don't know how many +100 hour characters I've made but it was worth every single hour. Because it was that good. I have a real hard-on for fantasy and what can I say? Goblins, magic, swords and beutiful landscape, this game has it all.
Battlefield 2 - When I think online FPS, I think of this game. I have never been a CoD-guy, aka pussys (just kidding guys =D) , I've always loved Battlefield. In to the year of 1943 fighting the japs online or kick russian in Europe, I'm with you DICE!
Portal - Really, everything that needs to be said about this game has alredy been said. Love the humor and the puzzels. Gabe Newell may eat my food any day.
Half Life 2 - "Rise and Shine Dr. Freeman, Rise and shine. In many ways, this might be the most polished game ever. There isn't a single gameplay flaw. Nothing more to say.
Mafia - I've always wanted to be a gangster. Okay not really, but I've always loved Gangster movies, such as GoodFellas if you didn't get that. Perfect story and excellent gameplay with the perfect length, god I love this game.
That oughta be the Top Ten, please don't make me choose between them.
Also, I want to mention Execution by Jesse Venbrux, you get get it free and legal here [http://www.venbrux.com/]. Download it and play it, play it twice.
You will know when you see it.