I would like to contribute to this thread as well with the following games:
Gothic II <- Far and above, the best RPG I have ever played, bar no comparison, no doubt. Story - gorgeous, world - superb, combat - great, immersion - complete. There has never been and will probably never be a more complete RPG that I, personally, could immerse myself into and just forget that time is passing.
Fable: The Lost Chapters <- Ok, so this is a bit left field. The game wasn't perfect, it had bugs, flaws, problems etc. But, back when it launched, it was the first game in a very, VERY long time that had managed to get me glued to my screen and which I could play for hours on end without pause. That meant something to me back then.
Ace Combat 5: The Unsung War <- The Demons of Razgriz shall never be forgotten!
Unreal Tournament <- Creative weapons, fantastic array of levels, great and fast paced combat...there is really nothing that could have been improved about this game except its graphics. Had they kept the formula intact and the feel intact for 2004 and 3, those would have ranked just as high as this...however, for now, this reigns supreme.
Demon's Souls <- Subtle storytelling that I've found to be so progressive and revolutionary that I feel like crying because almost nobody else seems to have noticed it. Maiden Astrea's part of the game could, by far, be considered one of the best and most intense moments in any game to date.
Fatal Frame <- My first survival horror. The best since. Fantastic feel, great graphics for its day, great combat, good story. A lot of games could stand to learn from this classic.
God of War III <- Yeah, I've found God of War 2 a bit boring in parts. Sue me!
Tom Clancy's Splinter Cell Chaos Theory <- Best one of the bunch with the best missions and most challenging scenarios. This is where the series should have ended, on the highest note imaginable.
Clive Barker's Undying <- Mood? Check! Story? Check! Graphics? Check! Crazy weapons? Check! Inventive worlds and creature design? CHECK! I miss this game.
Uncharted 2: Among Thieves <- Really? Do I need to say anything about this?
Metal Slug Anthology <- It's really hard to choose a favorite out of so many great games. The anthology is the best investment you can ever make if you're a die hard fan of the Neo Geo era of 2D platformers.
Need for Speed: Porsche Unleashed <- Best NFS game if you ask me, the PC version. Great locations to race in, great progressions, fantastic challenges outside of the main races. No other NFS game since has managed to grip me quite like this one...and I expect that not many will ever again. Is it so hard to believe that there are some of us that prefer racing in varied scenarios, without crowds of people on the side and without the ever present city city city landscape?
Jak 3 <- Combat + driving + a refined formula makes this the best Jak game in my experience.
The Elder Scrolls III: Morrowind <- Immersive is the name of the game. I hold it in the same high regards as Gothic 2, maybe just a bit lower as I've loved that one a bit more. But I've sunk more hours into Morrowind and have marveled insanely at just about everything it had to offer.
Dark Souls <- I prefer Demon's Souls from some points of view...but the refined combat of Dark Souls and the fantastic world and setting it has bring it on par with its spiritual predecessor.
inFamous 2 <- Bigger, better, more interesting, more colorful, this is the perfect sequel to one of the best games of the generation. Nuff said.
Journey <- Could I really say anything more about Journey that has not been said before?
And some games that came pretty damn close to this list (9-9.5...sorry for the long list).