I would recommend:
Darwin's World (I'm totally biased- I was a fan of this setting BEFORE I started working for the guys who put it out- but since now I do work for them, well, I *said* I was totally biased).
YotZ is awesome. Hardcore.
Earth AD2 was really cool as well. I love the PIG games and have several of them.
Gamma World: Again, I'm totally biased. They took the greatest post-apoc game world ever created, a game known for its awesome (though totally quirky) fluff and wonky rules, and then replaced it with a much less interesting world that also had wonky crunch (just wonky in different ways).
If it hadn't been called GW, I probably would have loved it, but it was. If your brain wasn't infected by GW 1st edition in the 70's like mine was, your mileage will probably vary.