For the Top 20 Best Games list Gamestate says:

We believe the following 20 games represent the pinnacle of videogame entertainment, though no doubt you'll have some very different views on the subject! Our own selection includes what we believe to be the best singular example from each given franchise, with an attempt to select our final list from as broad a range of genres as possible.


Of course the Top 20 Selling Games of All Time is pretty much what it says, based on the hard cold sales figures.