How about emotional impact? Amount of fun had while playing? How much the hype made you want to vomit on someone's copy?
How about we try number of man hours consumed while playing? Number of infants killed by neglect? Or how about a system where two games, of different genres, featuring different thematic elements, control schemes, histories, modes of play, and content are judged not against each other, as shooter versus fantasy epic, but as two kinds of fish, found in entirely different parts of the world, to be weighted against each other in terms of how well each goes with a lemon zest, and on no other principle?
Which is the better game, from the voice of the majority? The majority are playing two different styles of game entirely. This is not a contest. Independent sales of one hardly affects the sales of the other. Fans of CoD will get Cod, and fans of Skyrim will get Skyrim. Fans of both with most likely get both, over a period of time. How will they know they are fans, you chime? Because, in one, you shoot everything that moves, or just hang around and spam 'fix tank' for some unaccounted for points. In the other, you're a dragonslayer who can beat up a world, but get squished between the toes of a giant. You cannot measure these things against each other--they require two different units of unconvertable measure. Given at least a third component, e.g. number of near-naked men running around putting buckets on people's heads, you can compare the two. But alone, they are separate. There is no objective 'best' in terms of wholes, only a 'best' in terms of given comparisons.