What game should i buy?

XCOM, for sure!

Maybe you should consider buying one and making a giveaway. :X

maybe give something back? :3

i would pick wolfenstein

I just played Wolfenstein the other day and it was amazing. One of the best FPS games I've ever played. I haven't played any of the other games there, but I would most definitely recommend Wolfenstein.

DLC's for Darksiders don't add that much, If you really want to play it, then there's no need to buy the DE.
Max payne 3 without DLC's is a good choice.
Did you play X-COM:EU? Well Enemy Within is great, but it doesn't give you a new story, but rather adds stuff to the old one. So it's up to you, if you really need that.
Wolfenstein is an awesome shooter, you can't go wrong with that one.

I'd say you should buy wolfenstein and max payne, if you can.

Edit: ungh....
Edit2: meh....

I voted for XCOM, but it kind of depends. If you've played Enemy Unkown before, Enemy Within doesn't add a new story or anything, but if you were already planning on playing it again, it will be worth it, since it adds quite a lot to it.

Enemy Within only if you played and liked the base game of the XCOM remake. Enemy Within adds new elements to it to gain replay value. (Plus it makes the AI… cheat less obviously.)
Wolfenstein is a solid pick, one of the less casual shooters of recent times.