Ego! The largest, save for maybe Galactus, villain to ever be seen by mortals! And I know Galactus isn’t technically a villain, but anyone who tries to destroy the world should fall under that category.
Ego was a sentient planet that simply evolved into what it was. He can create super strong humanoids on his surface and also can travel at great speeds. He has a smaller planet, Id, that apparently left Ego to go find a party. His most common foes were Galactus, for obvious reasons, and Thor, for less obvious ones. I read up on the fights with him and Thor but didn’t really understand what was going on, but do think that part of it was Odin being an above-it-all prick just playing with us humans.
The first real crossover event that was published on its own (not in the pages of an ongoing series) that I bought was Maximum Security, an event about a group of aliens who wanted to blockade humans from spreading through the galaxy and did so by dumping a bunch of alien criminals on Earth. (I guess I was just drawn to Maximum events) Ego was one such prisoner and when he got to Earth he was very small and adopted by a farming couple in rural America (sound familiar?) but quickly spread into a growing mass that threatened to engulf the world.
With the help of some geniuses, like Mr. Fantastic (the world’s most humble man) and Iron Man, they were able to subdue the planet spores (or whatever) and trapped the essence in Quasar who could then never touch Earth’s ground. I’m not sure what ended up happening with that storyline, but I’m pretty sure it got out of him somehow.
In fact, I know it did, but I also saw Ego many years later in a Deadpool series. Although, it wasn’t an Ego story, but instead an Id one. Deadpool had gone into space and helped some aliens fight Id, who was going around destroying other planets. Deadpool blew him up.
I’m pretty sure that Ego was still around somewhere though. I seem to recall seeing him travelling around and trying to wake up other planets to talk to (or maybe that was in Hulk and the Agents of S.M.A.S.H.) Just a lonely guy I suppose. We’ve all been there. He’s still out there now, I’m sure, and I doubt he can completely be killed. Maybe he will even show up in the new Guardians of the Galaxy movie. Probably not, but just maybe.