5/5 yes
10/10 probably
100% no
THATS BULLOCKS! you might be saying.
It's not. If you are marking a game out of 100%, and you have 100 degrees that you can peg the game at, there is almost no way for a game to hit that benchmark. It's probably been done for some people, but other people will argue. All in all though, that doesn't matter. If Assassin's Creed did EVERYTHING I wanted it to, and I loved it more then god, and I decided to give it 100%, you saying 'No way' wouldn't make my opinion any less valid.
10/10 maybe.. well, again this is judging if you allow only whole numbers. If you allow for decimals, like 7.5/10, then that isn't actually 10 degrees, depending on how much freedom you have past the decimal, it could be a 100 degree system (allowing a single decimal place with 0-9 after it) to something even crazier (more decimal places). If you use only whole numbers, a 10/10 game doesn't need to be NEARLY as flawless and perfect as it does to warrant a 100% score. 10/10 and 100% in the world of reviews are two entirely different fish.
Now we get to the 5/5 rating system, which is actually the one I trust the most when it comes to rating systems. Again, if you allow a decimal place, you can increase the degrees from 5 to some arbitrarily high number, but most only allow for halves, similar to star ratings (like a 4 and a half star restaurant). If you allow for halves, then a 5/5 is basically a 10 degree system, soa 5/5 should occur about as frequently as a whole number 10/10, which is to say, not very often, but not "never". If you allow only whole numbers in a 5/5 scale, then that's only 5 degrees. A lot more games can afford a 5/5 rating, even games that have several large flaws in them. If they're fun and they're playable, they can have a 5/5.