Legion of Gold is the only Gamma World module I've ever used. Sure, I owned lots of them, but I never actually ran them as written. Instead, I just swiped a few ideas from them and incorporated them into my own campaign. Not so with module GW1, which formed the basis for one of my longer running Gamma World campaigns back in the day. Perhaps unsurprisingly, I have an inordinate amount of fondness for this particular adventure.
Written by none other than Gary Gygax himself (with the help of his son Luke and Paul Reiche III), Legion of Gold is a mini-sandbox in 32 pages. Set in a post-apocalyptic version of southern Wisconsin -- "Jen City" is Lake Geneva and "Horn" is Elkhorn, for example -- it consists of three "mini-adventures" and a larger location associated the eponymous Legion of Gold that's menacing the Barony of Horn. Like the best D&D modules, this one sets up a situation and then throws the PCs into it without any presumptions about how they will react or what the "right" thing to do may be. Characters are thus free to make their way as their players see fit, with plenty of material to aid the referee in handling it.
I know there are Gamma World fans out there who don't like Legion of Gold. They see it as insufficiently wild and woolly, which, for reasons I don't quite understand, is how many gamers view Gamma World. Certainly the game supports that style of play and there are plenty of bits in it that suggest a slightly less than serious approach, but, like D&D, Gamma World can be interpreted in a variety of ways, not all of them semi-comedic and/or over the top. For myself, Legion of Gold set the tone for how I've always played the game: post-apocalyptic pulp fantasy with science fiction trappings. I won't go so far as to say I treat Gamma World seriously -- how can you when there are humanoid rabbits that turn metal to rubber with a touch? -- but I try to treat all the setting's oddities not as opportunities for overt humor so much as occasions to emphasize just how weird the post-Fall world actually is.
That's the other thing I adore about Legion of Gold: the funky art, much of it courtesy of Erol Otus. If ever there were an artist born to draw Gamma World illustrations, it was Erol Otus. His work on this module unnerved me a great deal as a kid and, looking at even now, it's hard not to find the Buggems really creepy in that nightmare-come-to-life kind of way. Bill Willingham did some great work here too, although, as usual, it tends a bit more toward the "superheroic" than I prefer in my gaming art.
Regardless, I consider Legion of Gold a classic. Somebody needs to do something similar for Mutant Future. Jeff Rients did something close in issue 6 of Fight On! but I'd still love to see a full-bore module in the style of Legion of Gold. Guess I have another project to add to my list of things I never get round to doing ...