Halo Reach is the game that always get's the best turnout on game nights.
Agreed, which I think is mostly due to the great in-game party system is has. A similar party system was used in the Gears 3 Beta, so I'm hoping Gears 3 game nights will see some bigger turnouts.
As Failed said, Halo: Reach and Gears 2 are the current top contenders. Bad Company 2, Red Dead Redemption follow. Future releases like Gears 3 and Battlefield 3 will probably become popular. It usually takes a month or so after release for everyone to start voting for something new.