Collecting the Marvel Superheroes Roleplaying Game
Marvel Superheroes was the Marvel Comics universe contribution to superhero roleplaying games in the mid-1980s. This is the game with the power level descriptors like amazing, incredible, excellent, and so on. (An mnemonic device, FASERIP, helped.)
These adjectives confused the heck out of me as a teen trying to figure out what each meant and which was better than the other. Despite that little drawback, the Marvel Superheroes RPG remains a classic and collecting Marvel Superheroes is a must for fans of comic book roleplaying.
Marvel Superheroes Children of the Atom
Children of the Atom is the first X-Men resource, circa 1986. This includes the original X-Men (Cyclops, Jean Gray, Beast, Ice-Man, Angel) and the classic X-Men of Chris Clairemont's era: Cyclops, Storm, Colossus, Nightcrawler, Kitty Pryde, and (of course) Wolverine. If you want a single resource of Professor Xavier and his X-Men, this is it.
The Watcher's Book: X-Forces
This is an update on the mutants of the Marvel Universe for 1992. This was the time when the New Mutants (1983), X-Factor (1986), Excalibur (1987), and the New Mutants update, X-Force (1991), all introduced more mutants into the comics world, so there was a need for an update.
X-Men Roster Book
The X-Men Roster Book details the full roster of the X-Men up to 1998, when this book was published. It's natural that the characters that were big at the time of publication would be highlighted, and whatever odd story arc they were dealing with at the time would be reflected. I'm pretty sure this Wolverine is from the era where he didn't have an adamantium skeleton for time, which stinks. Otherwise, you'll get good details on most of the other essential X-Men.
Marvel Superheroes RPG Weird West
If you ever wanted to send your superheroes through a time warp into the Old West, this is your chance to make that happen. This is the second installment in a three-part time travel adventure which includes a trip to World War II and a chance to fight the likes of Genghis Khan, Napoleon Bonaparte, and Alexander the Great. I'm not sure what the story is, but the cover depicts a tyrannosaurus rex in a frontier city. Old West Marvel Hero additions include Kid Colt, the Rawhide Kid, and the Western-era Ghost Rider.
The Marvel Super Heroes SAGA System
I wanted to make one quick note on the Saga System and Marvel Superheroes by TSR. This game is NOT part of the Marvel Superheroes game line from the 80s, but is a 1998 diceless game which uses playing cards for resolution. I'll often see this listed among the other books on sale. It's a collectors item at over a hundred dollars, but it's not part of the Marvel Super Heroes RPG line.
Spores of Arthros
Campaigns that are heavy on the Fantastic Four or exploration-style superhero gaming are going to want the Spores of Arthros, which takes the heroes into the Negative Zone to face Annihilus and his minions. Includes everything from Negative Zone technology like the Cosmic Control Rod and a rare appearance by the living computer known as the Quasi-Motivational Destruct Organism--better known as Quasimodo.
Lands of Doom
Eventually, anyone campaigning in the Marvel Universe is going to have to face Dr. Doom. This module gives you all you need to pit your heroes against the greatest villain in the Marvel Universe--arguably the greatest villain in comic book history. Besides details on Latveria, there is information on Wundagore and its connection to both the High Evolutionary and Chthon, along with Symkaria, the homeland of Silver Sable.
Realms of Magic
Realms of Magic contains what you need to know about Dr. Strange and his rogues gallery, the rule of Sorcerer Supreme, and story elements like the Vishanti, the Octessence, the Ancient One, and the Eye of Agamotto.
Wikipedia has a list of all of the supplements for this game, so you can check it out if you want to put together a complete collection: List of Marvel RPG Supplements.