It's a simple question, or is it? How would you measure best? Complexity? Realism? Creativity? Detail?
I think it's fairly obvious that Tolkien has set the gold standard of all worldbuilding, but more recent authors like GRRM, Brandon Sanderson and JKR or the late Terry Pratchett have also created beloved worlds.
Some, like GRRM, are apparently more interested in complex worldbuilding itself rather than finishing their novels while others like JKR use the worlds more as a window dressing without keeping it fairly consistent. Is it alright if the Wizarding World is inconsistent if it serves the plot? How complex can Westeros become before it gets in the way of the story?
I think that GRRM and JKR are both extremes on the spectrum. When reading The Song of Ice and Fire, I felt like GRRM needed a proper editor to reign him in while JKR managed to build a fantastical world in 7 books which, upon closer inspection, makes no sense. On the other hand you have Terry Pratchett, who with the Discworld was clearly more interested in creating a parody of the real world, but still managed to make it very interesting and unique.