All right, brother, I'm gonna weigh in here. The greatest has definitely got to be Tolkien. While others have taken the reins and expanded the universes of Star Trek, Star Wars, and Marvel (much love to Stan the Man, he was my 2nd pick), Tolkien built the whole of his universe including multiple languages, and he did so in an age that required pure imagination. He crafted his universe using words and drew the readers in with those same words, enticing us to want to journey with Bilbo and the Dwarves to steal back the treasure from Smaug. Tolkien expanded his universe on his own, with no agenda of making bank from it, per se, but rather looking to entertain, delight, educate, and enthrall his audience. He was a wise man and he knew how to tell a story of good and evil, how to craft the words together, and he did all of that on his own. He also maintained a proper series "bible" and was consistent in all that he wrote, not expounding on or expanding after the fact with little tidbits like Harry Potter does, or having some inherent contradictions as found in Star Trek, or have an expanded universe that was canon and is no longer canon as is with Star Wars. His ideas are unique and contain within them the spirit of the human quest, unlike the sanitized fairy tales of Disney (read some of the original fairy tales, there are few happy endings in them). Tolkien established the fantasy genre to allow for the creation of things such as Dungeons and Dragons, and even giving way to George R. R. Martin's work. What remains is Pokémon and Dragonball, and while Pokémon can range from adorable to wicked, that universe lacks the character-driven stories and the depth of lore. And Dragonball... I'm reasonably certain that the whole of the DB/DBZ universe is based on a combination of acid trip and constipation - talking animals, aliens, and the fact that Dragonball and Dragonball Z are massive departures from each other, toss in the absurd martial arts training in gravity that should crush a human being into paté, and have you seen those guys try to power up in DBZ...?
tl;dr - Tolkien for the win!