Because others I respect have been writing about these, here is my current four in no particular order. Most have five, but I really one have four games I'd really be up for right now. I'm running the other one I'd be up for already.
1) Gamma World - using the setting from 1e/2e Gamma World, pulling partially from Metamorphosis Alpha and seen a little through the eyes of Paranoia.
Probably unfrozen special operations troopers (always a winner), or using an alternative method like the one suggests in The Best of Dragon 1, called "An Alternative Beginning Sequence for Metamorphosis Alpha" by Guy W. McLimore, Jr. of Microtactix, which basically has the PCs generated as clones dispatched to effect repairs. Either way, probably not the usual primitive tribesmen approach, although I like that too.
Obviously, I'd use GURPS. I think GURPS does Gamma World better than Gamma World did, having played the setting with both rules sets.
2) Dungeon Fantasy Gamma - As #1, but with a drop-in group of DF characters. If only to get the players familiar with Gamma World through something they are very familiar with.
It would have to be somewhat powerful DF guys, but wouldn't need to be crazy powerful. Plus, it would really be "depend on what is on your sheet" and not "depend on buying up replacement gear and consumables in town and come back later when the odds are better." You are stuck in Gamma World, deal with it. Use your magic to find out what those pre-loaded hypodermic needles do, and hope one heals 1d HP or restores 1d FP lost to spellcasting.
3) Immortals - our very long discussed but never run game with Immortal characters, run as episodes across history. Inspired by The Highlander and this article in Roleplayer magazine.
4) A short retro-clone campaign. S&W Complete, Labyrinth Lord, or Basic Fantasy. Just to remember what I like about those games and remind me of what I like about GURPS when we get back to it. Maybe even D&D, as in Moldvay, with d6 HP thieves because D4 HP thieves seems like a crazed slap to a class that barely is able to do what it does anyway. It's only +1 HP per level to all of the NPC thieves, anyway - easy enough to implement. Either that or a massive ramping up of their actual thief abilities, so they don't suck at their own job. One of those.