I want to say Uncharted 2, however I'm somewhat worried that only 2.5 million people have played the first, so that could have some effect on the 2nds sales, however Left 4 Dead is about the same, but a much shorter time on the market.
That being said, Uncharted 2 will probably have a great advertising campaign, boosting both the 1st and 2nds sales.
Put me down for Uncharted 2.