Jeeeezus... a whole decade? This has been a phenomenal 10 years in the gaming industry -- it's hard to narrow it down to just one...

For me, it's a 3-way tie between Metal Gear Solid 4, the Forza franchise, and the Elder Scrolls series (Morrowind/Oblivion)

Many, many, many sleepless nights with Forza and MW/Oblivion.