According the Game Rankings, Super Mario Galaxy is the best reviewed game ever. It beat out The Legend of Zelda: Ocarina of Time, Metroid Prime, Super Mario 64, and Resident Evil 4 just to name a few.
Source: http://www.gamerankings.com/itemrankings/simpleratings.asp?rankings=y
Source: http://www.gamerankings.com/itemrankings/simpleratings.asp?rankings=y