Which game should I buy?

Borderlands. Have you played Borderlands 2?

Bad Rats

?!? … LOL … At first glance, I thought the OP's avatar was a image of belted tighty-whities!

borderlands and spec ops

why not all of them?

Mafia II and Spec Ops The Line
If you thinking to get Borderlands then I recommend to jump straight to BL2 instead

I bougth Borderlands GOTY from Greenman nad 20% voucher code 3.99€, its very nice deal.

how can you buy 2/3 of them when there are four games listed

Mafia 2 and spec ops get my vote

Borderlands 2 imo - B1 Goty feels like most "go and kill" mmo

You should buy 2 Spec Ops the Line copies, get dissociative identity disorder and let both of your new selves play the game on separate accounts.

Mafia 2 + Spec Ops + Borderlands GOTY.

Mafia II

Borderlands has high replay value. I'm guessing Spec Ops does also, but I haven't played it.

And Mafia II is a fun game, if you like gangster related stuff. If not, foggetaboutit

MAFIA II

and crusader kings 2 complete ;)

MAFIA II (great story) and SPEC OPS: THE LINE (more great story)

These games are always on sale. For your information, they were on getloaded at any 2 for $10. So you just need to choose any one or two that you're sure you're going to play in the next two months. And in fact Steam Summer sales may have a even crazier bundle.

Darkness II, Spec Ops: The Line they both have great story. I haven't played much of Mafia II to tell you if its any good.

Spec Ops, Mafia 2, Borderlands Goty, Darkness 2 imho. Played all of them, and this is the way I would do it