Which game should I get? Civ5, Mafia 2, Spec Ops: The Line, or The Darkness 2?

I would say a lot of it depends on the types of games you like. Mafia 2, Spec Ops: The Line are both solid titles. The Darkness 2 is a fun playthrough as well, IMO. Not been sucked into the Civ world yet, so no opinion there.

Edit
I just glimpsed at your library, looks like the recommendations I made would be your best bet. Mafia 2 is similar to GTA, Spec Ops and Darkness are both decent shooters.

Just to be sure, you do know these are currently in a amazon bundle, right?

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Brother?

Mafia 2

I love you, space sloth!

CiV5 :)) great game! but i recommend you the gods and kings addon

For me Civ 5 is the best game in that list, you could get so many more hours out of it if you enjoy it.

Civ5 but only if you get it GOTY and with the Gods and Kings expansion, as it changes the whole gameplay for the better, better AI/Combat/Mechanics. Mafia 2 only if you're a fan of the first game, it's a great story-telling game. Spec Ops: The Line seems to have a great story, I heard many great things about it, but average gameplay. Darkness 2 seems to be very fun in all aspects.

Civ 5

Civ 5 is the best game on the list that offers the most hours of gameplay, but it really depends what you're looking for. If you enjoy GTA-style games the most, go for Mafia 2. If you like shooters, then Spec Ops: The Line and The Darkness 2 are both good games -- a bit short, but the stories are great. If you like strategy games or want to get into one, then Civ 5 is a great place to start, although like others said, you'd probably want to pick up the Gods & Kings expansion to go with it (it adds a lot).