I love Left 4 Dead 2. I have it for the Xbox 360 and the PC, and its much better on the PC. You can mod the characters to make them look like whatever you want them to, and mod the zombie skins to make them look different too. The common infected are Xenomorphs and the survivors are Predators in my game. It's just beautiful.
Actually, ArmA 2 had an amazing concept behind it, but it was just so outdated. ArmA 3 will be coming out soon though, and it will definitely be 110% better than the previous ArmA. It might become my new favorite very quickly. The videos have already convinced me to buy it. Now I just have to wait for it to come out.
GTA San Andreas or Cabela's Big Game Hunter 2012(well,I didn't play Cabela's BGH2012,but I saw it's a cool game).In San Andreas,you have limitless possibilities,you can drink juice and eat chocolate bars,travel to more different places,cool missions,good graphics and you can play mini-games,unlike Vice City,where you can only fight cops and finish missions..It's a difficult choice...