(Is not) is a question in negative form, so, the answer is (yes); meaning, it has been proven that there is (no most just of judges) except Allah (swt). Allah (swt) does not ask a question, but affirms a fact, and He (swt) wants this affirmation to come from them by saying (yes) because this matter is undeniable, and (Yes) makes the negative imperative.
Whenever you find a plural noun in which Allah (swt) describes Himself with along with His slaves, you know that this attribute is applied to them too, however, Allah’s (swt) attributes are eternal whereas, people are changeable, subject to strength and weakness, changes.
Hence, (the most just of judges) will judge between you and the deniers, and Allah (swt) is (the most just of judges) in all what He (swt) created and made, this indicated His (swt) knowledge and wisdom (glorified is He). Abo Horayra (may Allah be pleased with him) reported that:
“Whoever recited “By the fig and the olive” Then he recited: ‘Is not Allah the best of judges?’ then let him say: ‘Of course, and I am a witness to that.’”
And we are all witness to that.