My problem with most PA settings are they seem totally without goals. Only the blanket one of survival. But if the world is dying with no come back from the brink which is a popular theme then what's the point. I save the village but hey the spreading radiation is going to get them anyway.

I like Rifts. I am not keen on the magical or superhero elements but the game designers did say GM's should pick and chose. It has a lot of flexibility and can be hi-tech which I enjoy.

Book wise about the only PA ones I really enjoyed was a series called the Amtrack Wars by Patrick Tilley. Which was a hi-tech country who lived underground fighting against the surface Mutes. It has some cool twists and surprises so I don't want to mention them here in case anyone gets a chance to read them.

But if I had to choose a RPG and only one i'd say Rifts because it has tremendous flexibility. The Coalition can be good guys or Nazi's. It can be a hi-tech war against aliens, or you can be a resistance force fighting against the machines kind of like terminator. You can include giant mechs or not. Be a merc unit, a traveling knight kind of character. There is loads for a GM to take and make his own. Tons of classes which a GM can pick from and again the game designers have encouraged GM's to include/exclude ones that meet or don't the setting.