As I think of it, a story in X-MEN 107-108 (the last Claremont/Cockrum issue, the first Claremont/Byrne/Austin issue) has the X-Men fight a group of heroes in the service Princess Lilanda (of the Shi'ar race). The group of heroes, the Imperial Guard, are (like Squadron Supreme were the JLA in AVENGERS) in all but name DC's Legion of Superheroes!
These pre-crossovers, before the ACTUAL Marvel/DC crossovers, were really fun.
