The very first and, in my humble opinion, best Marvel RPG ever was the D100 system. This game was called Marvel Super Heroes. It is now referenced as the "classic" Marvel RPG.
As you can see from this cover, this system was published by TSR. TSR took full advantage of its Marvel license putting out dozens of products that sold very well from 1984 into the early 90's. Eventually Marvel and TSR parted ways for whatever reason (probably Marvel wanting too much money for licensing rights - Marvel was going through a bankruptcy at the time).
While the core resolution is one of the best ever devised (it is simply and very flexible to fit all situations), one of the biggest drawback to this game is the increasing complexity the rule books went through. The old rule books were usually never revised or updated to reflect additional rules complications or clarifications. Originally, this game was released as a "basic" set in which players could basically just play their favorite heroes from the comics. Rules were very simply. When this proved to be popular, TSR released the Advanced Set - Kinda like a version 2.0. But where things get complicated is when you consider other sourcebooks that expand greatly one particular aspect of the core rules - say, super powers or magic. These "enhanced" advanced rules are not reflect in TSR published character write-ups or adventures, making things unnecessarily complicated at times. To add to this, like a lot of RPGs published in the 80's the rules aren't always as specific or clear as you would like them to be.
But, all nit-picking aside, this is a fabulous system and an extremely fun world to play in. I have been using this system for decades and I own an extensive amount of material for this system. TSR really did explore every aspect of the Marvel U - there are literally hundreds of comic book characters that had officially published character sheets sold for them. Some popular characters even had multiple different versions of the same character released (say, Spider-Man as a teen, Spider-Man with six arms, Spider-Man as an adult, etc.). What's better is that these sheets were perforated and three hole punched so you could store them in binders and organize them as you wished.
You can find a plethora of material available on Ebay and other online sources still to this day for very reasonable prices. If you're a fan of the Marvel Universe and like RPGs, I STRONGLY encourage you to purchase the core "advanced" rules for this game and give it a shot. I promise you won't be disappointed.