For me, Mutant: Year Zero is the best post-apocalyptic game on the market right now and it's well augmented by spinoff games. Gamma World has some interest, because of its venerable status, but there isn't a current edition out right now (although POD copies of previous editions are available). Judge Dredd has post apocalyptic elements (Cursed Earth) although I'm not a fan of the system (particularly character generation). Apocalypse World is, in my view, one of the weaker games in the 'Powered by Apocalypse' group of games, if I'm being honest, although it has it's fans. If you want to go more left field, then Paranoia is a post-apocalyptic setting, technically speaking. So is Low Life for Savage Worlds. So is Eclipse Phase. And yes, technically you could count All Flesh Must Be Eaten, as with other games like the End of The World line by FFG.