I'd hop for the one with the greatest probability of having post-apocalyptic bunnies. Seriously though, zombies are relatively boring. Nuclear is rather sterile in multiple ways. Aliens... which ones? There's a lot of them, and not all make a pleasant apocalyptic experience. I wouldn't want anything otherworldly to burst out of my chest, perform surgeries or experiment on me, or violate my dignity in any other way. Honestly, whatever happened in Metro 2033 would be interesting. It wasn't just nuclear, there was some mad scientist bulshittery there. And before anyone points it out: Yes, I know what the metro in Warsaw looks like.