My first RPG was AD&D (though it could have been OD&D, I guess). I was a kid, my mom and her boyfriend went out to dinner with neighbors, leaving their teenage son to babysit me. He had me roll up a fighter (IIRC, he had 4 hp), and then my guy headed out for this big city that he had this amazing map of (I think it was the City-State of the Invincible Overlord). My guy got beat up by a city guard and had his arm broken, and I was hooked. Once I found an AD&D book at a toy store, I pestered my mom until she bought it for me.
I think it was the Monster Manual, rather than the Player's Handbook, amusingly enough.
My all time favorite campaign is one I'm playing in now, a Mutants & Masterminds 2e game. At 10:20 am on October 12, 2015, the spacetime continuum ripped open near Reno, NV, and transdimensional Nazis start invading, with grav tanks, supers, sorcerers, bodyjumpers, and millions of enslaved soldiers poised to cross over. However, a band of guerilla fighters sets off a nuke in the middle of the portal, causing a cascade of energy to spread across the world, and suddenly some people started developing superpowers. Meanwhile, the recon squad of superpowered Nazis that made it through are trying to continue their mission of preparing for the inevitable invasion; there's a parallel nightmare dimension that's leaking across into the Earth, spreading darkness, despair, dread, and doom; and some people with powers decide they want to conquer the world or destroy it. Oh,, it also turns out that all this has happened thousands of times before, with each iteration ending in the death of everyone in the world, before the cycle starts over again. Only one (well, probably two; sometimes three) people remember what happened before, and try to figure out how to stop it all. Too bad the accumulated trauma of all the deaths they witnessed (and committed) seem to have broken them, and now their primary goal is "stop the Nazis from winning, by destroying the world before they can conquer it". The PCs job is to survive, stop the invaders, save the world, break the cycle, and maybe try to live.