You reckon? I'm not trying to start a fight here, I'm honestly interested. What, GTA, COD, Mario, Zelda.... Elder Scrolls maybe? That's about where I start to pull a blank.
Since 2001, when Halo released:
For reference, Halo sold 5m, Halo 2 sold 8m, and Halo 3 sold 11.5m. Halo Reach sold 9.1m. Halo 4 is apparently around 4 million. Source for all is Wikipedia. Total sales are around 39m.
On Wii we have:
Wii Sports (81.64 million)
Mario Kart Wii (34.01 million)
Wii Sports Resort (31.54 million)
Wii Play (28.02 million)
New Super Mario Bros. Wii (27.61 million)
Wii Fit (22.67 million)
Wii Fit Plus (20.48 million)
Ignoring Wii Sports, we have Mario Kart Wii that has almost matched the entire Halo series alone. Wii Fit series combined is bigger. NSMB Wii is bigger than all the games combined minus Halo 3. Wii Sports Resort is bigger than the series minus Halo 2 numbers.
On Nintendo DS:
New Super Mario Bros. (30.18 million)
Nintendogs All versions (23.64 million)
Mario Kart DS (22.57 million)
Brain Age: Train Your Brain in Minutes a Day! (18.96 million)
Pokémon Diamond and Pearl (17.57 million)
Brain Age 2: More Training in Minutes a Day! (14.83 million)
Pokémon Black and White (14.71 million)
Pokémon HeartGold and SoulSilver (11.90 million)
Animal Crossing: Wild World (11.51 million)
Every game on the DS list above has sold equal to or more than Halo 3, Animal Crossing being the equal to. That is ignoring the Pokemon series combined sales.
Call of Duty games:
CoD 2: 2 million (as at 2008)
CoD 3: 2 million (2008)
CoD MW: over 10 million (2009)
CoD MW2: over 22m (2011)
CoD MW3: over 9m (2011)
CoD W@W: over 11m (2009)
CoD BO: over 25m (2012)
CoD BO2: over 7.5m (2012)
Playstation 3 games:
Gran Turismo 5 (9.19 million)
Playstation 2 games:
Grand Theft Auto: San Andreas (17.33 million approximately)
Gran Turismo 3: A-Spec (14.89 million approximately)
Gran Turismo 4 (11.6m)
XBox 360 games:
Kinect Adventures: 24m (pack in with kinect)
GTA 4:
Over 25 million (2012)
World of Warcraft: consistently over 10 millions subs since launch year
Minecraft: over 9 million (2013)
Roughly speaking, Halo is a big name but it has been dwarfed by other series, especially when you start adding together Mario Kart series, Wii Fit series, Wii Sports series, Mario Galaxy series, NSMB series, Call of Duty series, Gran Turismo series, World of Warcraft and all add ons, The Sims, Nintendogs series, Animal Crossing series, Grand Theft Auto series.
Halo is big, but not that big.