GAMMA WORLD - especially 1e. It's Jim Ward's post-apoc masterpiece, definitely deeply inspired by his Metamorphosis Alpha work. In many ways GW is MA on Earth. I can't accurately describe how much I've been inspired by this game throughout the decades. It's so concise, so complete and so rocking with creativity that I've rarely seen anything come close. Yes, you can emphasize the gonzo into the realm of silly if you wish, or you can go grimdark post-apoc as well.
D&D4e had a version of GW that's quite good. Yes, the collectible cards and the silly bits are dumb and should be ignored. Yes, the GM needs to reign in chargen if you want a more serious game. But that said, its far and away the best iteration of the 4e ruleset and works great (and quite deadly) as a skirmish RPG / boardgame hybrid. I highly recommend it anyone who enjoyed 4e (hello lurkers!).
MUTANT FUTURE is the retroclone of Gamma World. It's build on Labyrinth Lord and I haven't run it RAW, but I've stolen bits and pieces because there's lots of good ideas within. Here's the free no-art download.
http://goblinoidgames.com/index.php/downloads/WASTE WORLD was GW author Bill King's version of 40k+Gamma World hybrid (with Dune sprinkles!). Sadly, its out of print, but I can heartily recommend it based on multiple campaigns and numerous one shots of actual play. Chargen is point-buy and mechanically sound so its hard to make a bad character or an overpowered one. Moreover, the factions and archetypes in the setting are described broadly, swiftly and excellently so its easy to pick chargen options. Copies are regularly on eBay for $20. Worth every dime. I'm happy to answer more questions about WW if anyone wants to know shiznack.
Here's a RPG.net review from 2008.
https://www.rpg.net/reviews/archive/13/13937.phtml