Capcom's well-received line of licensed/crossover 2D fighters, commonly referred to within the community as the “VS” or “Marvel” games. Excepting MVC2, which was originally released on the Naomi, all reside on Capcom's CPSII hardware natively.
They are largely responsible for at the very least proliferating, in the cases where they did not outright establish, the ideas that have come to define a “modern” 2D fighter (Command Launchers, Aerial and Ground Chain Combos, Beams, Tag-Team gameplay, Infinite Juggles, etc). More is more, and all this jazz.
As far as interesting intellectual tid-bits go, well: These games are possibly the finest examples of a concept that runs throughout many others in genre: the gameplay dynamic that emerges is largely beyond the scope, and many times obviously against the intent, of the games' developers.