Well since MW2, only a handful of Game modes there are which people actually join, I think 3 on average...
They are in order of popularity;
Domination,
Mercenary Team Deathmatch,
Team Tactical,
Team Deathmatch & Search and destroy are tied because they do get people trying to play them but rare that they actually start... Out of the two, Search is best to join that normal TDM.
The others are VERY quiet but Old school does come on a bit but it's a fail mode and didn't deserve being on the list xD
Hope i helped!