If you are talking about the largest environment, I think that has to be No Man's Sky. Minecraft is also technically infinite... However, those are procedurally generated.
I think you really want to know what game has the largest actual map, created with the usual level-creation process. According to what I know, it has to be an older game, Bethesda's Daggerfall: Elder Scrolls 2. Reportedly has a map size of over 160K square kilometers.
