Major Nelson has released some interesting stats based on what people connected to Xbox Live have played and bought during the year (remember that when looking at the unique user lists - not everyone is connected with their consoles - oddly enough). Still it represents the closest we will get to the truth of what people are actually playing on their Xbox 360's.
Xbox 360 Top Live Titles (based on unique users)
1 Halo 3
2 Call of Duty 4
3 GTA IV
4 Gears of War 2
5 Gears of War
6 CoD: World at War
7 Guitar Hero III
8 Battlefield: Bad Company Demo
9 Star Wars: The Force Unleashed Demo
10 Rock Band
11 Fable II
12 Mirror's Edge Demo
13 Madden NFL 09
14 BF: Bad Company
15 Rainbow Six Vegas 2
16 UNO
17 Castle Crashers
18 COD: World at War Multiplayer Beta
19 Forza Motorsport 2
20 Too Human - Demo
Top Arcade Titles (Full Versions purchased)
1 Castle Crashers
2 Geometry Wars: Retro Evolved 2
3 Braid
4 A Kingdom for Keflings
5 UNO
6 Super Street Fighter II Turbo HD Remix
7 Fable II Pub Games
8 Duke Nukem 3D
9 Bionic Commando: Rearmed
10 Worms
11 SOULCALIBUR
12 Portal: Still Alive
13 DOOM
14 1942: Joint Strike
15 Bomberman Live
16 MEGA MAN 9
17 N+
18 Wolf of the Battlefield: Commando 3
19 Marble Blast Ultra
20 Ultimate Mortal Kombat 3
Original Xbox Top Live Titles (based on UU's)
1 Halo 2
2 Fable
3 Star Wars: Battlfrnt 2
4 Counter-Strike
5 Fable: Lost Chapters
6 Doom 3
7 Conker: Live Reloaded
8 Splinter Cell Chaos
9 SW: KOTOR
10 SW: Republic Commando
11 Star Wars: Battlefront
12 Burnout 3
13 Guilty Gear XX #Reload
14 Gauntlet Seven Sorrows
15 Call of Duty 3
16 Ninja Gaiden Black
17 Crimson Skies
18 MechAssault 2
19 Call of Duty 2
20 Rainbow Six 3
Source: Major Nelson