This is tough to answer as my tastes change and video games don't generally hold my attention as long as they used to. I played World of Warcraft for just under three years though and quit, likely for good, last November. Clearly, that was my favorite computer game during that time.
I played the snot out of Fallout 3, but was left wishing for community created content like Oblivion had.
I've started this year dusting off the old xbox 360, though, so I'm not playing much on the computer at this time. Currently, I'm waiting to see how
Star Wars: The Old Republic turns out. My son, Byron, is highly anticipating that game as well.