The absence of statistics and studies on video game sales makes our task difficult. We're basing our rankings on the numbers published by VGChartz and Wikipedia. The problem is that those resources only count the number of physical copies sold, leaving out online sales. Furthermore, VGChartz doesn’t break down sales for Windows, Mac, and Linux. And finally, it’s sometimes impossible to distinguish between PC and console sales because of the vague statistics discussed by game publishers.
So, in order to create the most accurate ranking possible, we tempered the results from those publications by referring to other rankings and taking each game’s cultural impact into consideration. That’s why, instead of calling this a list of best-selling PC games, we're working with the most popular PC titles, sorted in alphabetical order to limit the subjectivity of our approach.
We also made the decision to limit ourselves to one title per series in order to present more games and avoid the inevitable redundancy. For instance, The Sims is not included because The Sims 2 sold many more copies. There are also series that left their mark on video game culture, such as Call of Duty and Grand Theft Auto. But each title from those families sold considerably less than the games in our list, which explains their absence.