I would say either Skyrim, Fallout: New Vegas or Mass Effect 2.
The first 2 because, well, you can be playing both for a month and not have them completed(I garuntee(spelling?) that my first 6 hours in it will be diiferent to yours).
Mass Effect 2 because speedrunning it you could get 2 playthroughs out of it, but it's the choices you make that makes it great (I've had it on PS3 since March and I still play it) that, plus you get nearly all the DLC on the disc with the PS3 version (so that's 3 armor sets, multiple new weapons and 2 story-bridging DLC, that and two new fully usable squadmates available straight after the ~hour long intro).
The first 2 because, well, you can be playing both for a month and not have them completed(I garuntee(spelling?) that my first 6 hours in it will be diiferent to yours).
Mass Effect 2 because speedrunning it you could get 2 playthroughs out of it, but it's the choices you make that makes it great (I've had it on PS3 since March and I still play it) that, plus you get nearly all the DLC on the disc with the PS3 version (so that's 3 armor sets, multiple new weapons and 2 story-bridging DLC, that and two new fully usable squadmates available straight after the ~hour long intro).