As Squilliam said, its based on sales. Out of those pc games only half-life 2 and css would have sold enough to be on that list (css mainly because it was originally bundled with hl2). Im a big advocate of pc games but crysis just isn't in the same league in regards to sales.
Cod 5 wont sell as much as cod 4 did imo. R2 and kz2 will sell well but they wont sell enough to be on that list either. Cod 4 and halo 3's positions are secure in my opinion. Both gears 2 and cod 5 have a shot at entering the list though.







