Not sure if this was about games released in the last decade, but I'm listing the ones I played this decade.
Top 5
Metal Gear Solid series - the interactivity of these games blew me away (MGS and Sons of Liberty mainly)
Gran Turismo Series - love cars, tuning them, and racing them.. one of the best simulators
Battlefield 2/2142 - spent countless hours playing these two.. mostly BF2
Freelancer/Disco - getting close to being the most time spent on a game ever
Rockband series - I play guitar and drums, so this was a ground breaking game.. being able to jam with friends
1-) Baldur's Gate II & Throne of Bhaal (2000 - 2001) - Best game ever. Also has TONS of mods.
2-) Deus Ex (2000) - Probably the best storyline in a FPS/RPG and ambience. Great cyberpunk / dystopia
3-) Diablo II and exp. (2000-2001) - Best hack'n'slash game ever. Also the rune trade and ladder stuff gone forever on battle.net (I was very active there till 2007)
4-) Half Life II & expansions (2004 - 2008) - One of the best sci-fi scenarios ever written for a game.
5-) Undying (2001) - Still the BEST frightening FPS even with its outdated graphics.
Also, not my top 5 but I should praise these games;
- Empire Total War - for being a great strategy game
- Guild Wars - for proving that F2play also works with mmorpgs.
- Kotor I & II - proving that if wanted, a good Star Wars game can be made after 1997
- Eve Online - proving that space sims STILL SELL WELL after the Origin era.
- Pro Evolution Soccer 2006 (2006) - Probably the best football game. Fun - Replay value - modding and difficulty balanced well. All other games (including other PES games) has a flaw that destroys the "fun value".
Compilation of Final Fantasy 7 (DoC / CC ) - Im an FF7-a-holic so anything related to the series (minus Kingdom Hearts that horrible, horrible insult to all things Final Fantasy) gets a win from me.
Diablo II / DII LoD - This game, no matter how many times I beat it, still seems interesting somehow. Especially with the unlimited stash. I made a hobby of collecting every set and unique item. Needless to say that isnt done yet.
Starcraft / SC BW- I grew into RTS games with this one. I like it and wasted many an hour of my life perfecting Terran swarms of doom.
Freelancer / Disco 4.85 1st release - I really like FL and I liked the first release of .85, but with alot of the changes after the initial release I find the mod becoming a bit less fun with every release.
Call of Duty Modern Warfare II - Best FPS I have played in a long time. I like the aspect of having to unlock things and getting rewards for doing insane stuff. It adds a level of fun to the FPS environment.