Dang, a lot of replies already, but i dont think its the greatest open world, i loved the game, for me its a 5/5 masterpiece, but honestly i dont feel like replaying it these days, i just lost the motivation, its too big for its own good, again...i love the game and i enjoyed it immensely, i put more than 250 hours in it, but i can't bring myself to do another run, it just feels tedious to clear the world, when it comes to replay value i prefer linear games, however in the defense of Elden Ring i got a lot of hours from it just by 2 playthroughs, so i think there was the tradeoff you lose few runs for the amount of bigger 1 run.
As for the open world itself, i think Bethesda Game Studios are the best at making engaging open world experience, after them comes Rockstar, then its up for grabs, Elden Ring, BOTW are well made open world too, Gothic is another option, Witcher 3 is for entirely different reason, but i wouldn't put any of those as a pioneer in open world, the only ones imo that can enter such discussion are Bethesda games, Rockstar and probably Breath of the Wild...Probably... even though Elden Ring is the better overall game.