unless the numbers on steamcharts are wrong, World at War still has pretty high player counts, more than AW and Ghosts certainly, I keep meaning to see if it's true in multiplayer
EDIT: 30 day averages
BOIII - 6700 (no sp/mp split)
BOII - 1775
WAW - 1625 (no sp/mp split, Steam not required)
MW3 - 1564
MW2 - 1538
BO - 629
Bad Company 2 - 500 (Steam not required)
BOII Zombies - 470
Ghosts - 365
4:MW - 257 (no sp/mp split, Steam not required)
AW - 211
2 - 122 (no sp/mp split, Steam not required)