Damn y'all babies lol.
He conquered the grand line. Which included, but is not limited to, reaching Raftel. (The most elusive / hardest to reach location within the New World portion of the grand line.) It likely also had to do with him "making nice" with a variety of his 'enemies'; similar to the story we see playing out for Luffy.
Anyone can reach Raftel, it's just an extremely arduous journey and his crew was the first / only so far to make it. What makes the Pirate King now? Finding the One Piece! (Fabled to be Found in Raftel!)
If you find the location of the four Road Poneglyphs and map the four islands they lead to, Raftel is supposedly within the Center of the intersections. (Which form an X itself.) However, if you take with you the knowledge of the Poneglyphs you can also discover the Rio Poneglyph after reaching raftel!