Eh . . . exactly one month? That's hard to recommend without knowing your playing habits.
I could say Arkham City, Deus Ex: Human Revolution, or the Metal Gear Solid HD Collection if you only play for a few hours each week. Or even Mass Effect 2, I put 60 hours into it on my first playthrough without even completely finishing it.
Skyrim is a bit iffy because you may get the lag bug that happens after save files get too large, but you may not, and if you only play for a few hours each week it would have well more than a month's worth of content.
If the reason for a one game only deal here is a $60-80 budget, then if you could find Metal Gear Solid 4 used for pretty cheap I'd say get the HD collection and that, because (if you're a retro fan and play the Metal Gear and Metal Gear 2: Solid Snake games contained along-side MGS3 in the HD collection) you'd be getting six games there.
I could say Arkham City, Deus Ex: Human Revolution, or the Metal Gear Solid HD Collection if you only play for a few hours each week. Or even Mass Effect 2, I put 60 hours into it on my first playthrough without even completely finishing it.
Skyrim is a bit iffy because you may get the lag bug that happens after save files get too large, but you may not, and if you only play for a few hours each week it would have well more than a month's worth of content.
If the reason for a one game only deal here is a $60-80 budget, then if you could find Metal Gear Solid 4 used for pretty cheap I'd say get the HD collection and that, because (if you're a retro fan and play the Metal Gear and Metal Gear 2: Solid Snake games contained along-side MGS3 in the HD collection) you'd be getting six games there.