Gears of War was the first game of mine to really showcase how far this gen could take us technologically. Me and my friends played ranked Warzone all the way to "Seriously..."
CoD 4: MW I spent a great deal of time on. IW really got the formula right to keep you addicted.
Dragon Age: Origins brought me out of my 3 year WoW slump. I've always compared the battle system to a 5-man dungeon in WoW, except you control everybody. The story and characters were great, the battle system challenging, leveling system really shined too.
Best - Demon's Souls. The only modern style game I've played that felt like an old school one. Die. Next life. Learn. Succeed. Until you're good enough to get through the whole game without dying. Dark Souls is almost as good, but didn't come first, and I still like Demon's Souls better anyway.
Runner up - Oblivion. I don't care if Skyrim is better from a technical standpoint. Oblivion sucked me in for a solid six straight months back in 2007 and it never got boring. I lost interest in Skyrim when I wasn't even that far in, although to be fair, it may be because I'm a little older now and my tastes have changed.
Your name is sung and tattooed now on my heart - here I will carry you forever CarryTori Amos