1-10, or even 1-5. Anyone who pretends to be more specific is full of crap. The reason is this: there's a margin of error in any scoring system, and a rather large one in game reviewing. To claim that "Grand Theft Auto is 3 percent better than Zelda," as a 100 point system does (I'm making these numbers up) is completely ludicrous. You can't measure the worth of a game that precisely, and you certainly can't use those scores to compare between genres. The best you can do is say something is "bad," "average," "good," or "exceptional," to within a margin of error of maybe 10%.
If a site like Gamerankings or Metacritic uses a 100 point system, they're the ones who should change, not the reviewers who use a realistic system.