In my opininon Civ IV is a brilliant game, but definitely not the best game ever, even in the strategy genre it has massive rivals in the form of Total War, Europa Universalis, Hearts of Iron etc. And if you look at computer games as a whole, there is the Final Fantasy series, the earlier Command & Conquer games (latest ones don't cut it IMO), countless brilliant FPS's and MMORPG's like World of Warcraft and Guild Wars.