It is indeed God of War 3. I've checked the list, and none of the ~ 20 games above it on the list lack multiplayer.
You really raise a quite interesting point, though, OP. Just look at the Wii: all the 11 best selling games have multiplayer. The first non-multiplayer game is Twilight Princess, at 5.78 million. The next one after that is Super Paper Mario, the Wii's 25th best selling game at 2.97 million.
Only one game lacking offline multiplayer has made 5 million on the Wii.
On the X360, the best selling non-multiplayer game is Assassin's Creed at 4.99 million, the X360's 11th best selling game. Then it's Assassin's Creed 2 at 13th place, then Oblivion at 20th place.
Only those 3 games out of the top 20 on the X360 lack online multiplayer of some sort.
On the Ps3, it's Final Fantasy 13, its 6th best selling game, followed by Assassin's Creed 2 at 9th spot, in front of Assassin's Creed as 12th. It then has God of War 3 on 18th place, in front of Uncharted 1 at 19th place.
It seems that online multiplayer is near mandatory on the X360, and that offline multiplayer is near mandatory on the Wii. The Ps3 doesn't seem to have the same requirement for that.
It's also quite remarkable that only three console games without multiplayer have made 5 million this generation. The Legend of Zelda: Twilight Princess, and Assassin's Creed 1&2.
