According to Torrentfreak, CoD: Black Ops is got the dubious distinction of being the absolute winner in terms of illegal downloads. It is estimated that 4.27 million PC users downloaded an illegal copy of the game as of December 26. Bear in mind that the game launched in November and that the number of total illegal downloads will continue to grow for months to come.
However, the title sold 5.6 million copies in the first day of sales and netted revenues in excess of $1 billion within weeks of the launch, making it the most successful game launch in history.
Battlefield: Bad Company 2 ranked second with 3.96 million illegal downloads, but it launched back in March. Mafia 2 came in third with 3.55 million downloads, while Mass Effect 2 and Starcraft II ended up with 3.2 and 3.1 million illegal downloads respectively.
Love them or loath them, people who download pirated games appear to have pretty good taste, although I personally believe Civilization V was the best game of the year, like every previous Civilization title.