Well if you mean in terms of sales...If its going to be across all platforms then likely Black Ops 2 will evidently sell the more copies than the other 3. Followed by GTA5 and then AC3 will be close with Halo 4 considering Halo 4 is only on X360.
Lets look back for a moment on rough sales figures of each franchise:
(2011 Release)MW3 across all consoles and PC sold ~28 million
(2010 Release)Black OPs across all consoles and PC sold ~27.3 million
(2007 Release)Halo 3 on the xbox 360 has sold ~11.5 million
(2010 Releaes)Halo:Reach on the x360 has sold ~9 million (however with digital sales this is higher. maybe ~10 or 10.5 million but not sure)
(2008 Release)GTA4 across all consoles and PC sold ~19.5 million
(2009 Release)Assassins Creed2 across all consoles and PC sold ~10 million
(2011 Release)AC: Revelations across all consoles and PC sold ~7.3 million (the series seemed to have lost some steam after AC2)
Now with how each series seems to perform I would put my predictions somewhere around this ball park (no one can be sure but I would say I'm close):
(2012 Release) Halo 4 only on x360: ~13 to 14 million lifetime(alot of people say lower due to end of gen, but just look at preorder numbers right now. Way ahead of Halo:Reach numbers + It has Master Chief who has been absent in new storylines since 2007 in Halo 3. Trust me regardless of nearing end of gen this will be huge and at the very least will sell 10 million+)
(2012 Release) Black Ops 2 across all platforms: ~29+ million lifetime (many people think one of these times Call of Duty will begin to slide...well trust me it wont be this one. The Zombies mode is more popular than ever and with the devs saying they are expanding it in immense ways this will draw the zombie fans like milk to a hungry infant. Plus the inclusion of Strike Force mode which effects campaign outcomes and being able to make different choices during the campaign will get some big thumbs up. Also it looks like they are doing a lot with the multiplayer. Like it or not this game will again be the biggest selling yet.)
(2012 Release??)GTA5 across all platforms: ~20+ million lifetime EDIT: ~19 million lifetime if 2012; ~14 to 15 million if 2013 (This one is a bit harder for me to judge as a big factor is release date. If it dosn't get released until 2013 then lifetime sales will be less and may be slightly EDIT: Likely much lower than GTA4 due getting into the new gen. This is a hard to predict animal. It will definitely be a huge seller as always. If it releases in 2012 then Im confident EDIT: I think it will sell around or maybe a little more than GTA4. There is alot of enthusiasm for it. EDIT: After considering its release time even if it releases in 2012 Im not sure it will reach the sales of GTA4 but I do think it will be close.)
(2012 Release)AC 3 across all platforms: ~13+ million lifetime Edit: ~13 to 14 million lifetime maybe more (This game is only gaining in popularity and in chatter across the gaming sites. A game set during the revolutionary war let alone making it an Assassins Creed title!? Sets the stage to have huge sales and Im confident it will be the most sold of the series yet especially remembering it will be on the Wii U platform as well. I can barely count the amount of games set in the Revolutionary War, but to have it be an Assassins Creed title will draw huge sales and will be a very fresh change of setting to the series that is overdue.)
So these are my predictions for the 4 games. Hope you enjoyed reading my book post :)
