I assume the question is which composer would be the best to write new scores in Williams’ established Star Wars style. Not only does such a composer need to have the skills to write in this particular style, but also be willing to do that for the entire score while prioritizing the Star Wars style over his/her own style. I think McNeely has shown he’s not up to the task with Shadows of the Empire. I also doubt Desplat’s able, judging by his Harry Potter scores.
I would say the best qualified among the established Hollywood composers are Harry Gregson-Williams, David Arnold, and John Powell. It will be interesting to see whether Powell succeeds with Solo!