Loaded question that.

Personally, As you have a lot of free time I'd go with Skyrim, I've put near on 200 hours into it, I've had to take a break! But mainly because GT5 have released DLC. But it is an emit game, but not massively challenging, particularly of you put the time in completing side quests and level up you chatecter.

Haven't played New Vegas, but hear mixed reviews about it.

But if your into a Challenge then Dark Souls is the way to go, it's properly hard, but for some reason people are saying that despite the difficulty it's rewarding to play.