Student or Learner
An only exceptinal person.
Only an exceptional person.
However, neither makes the grade as a complete sentence.
"Who qualifies to command a Mars mission"?
"Only an exceptional person". Meaningful; a complete thought.
Only an exceptional will receive the best services. Is it ok?
It might become necessary for those who use the logically absurd, "one of the only".
A: "We can't fire Tim. He's one of the only exceptional people we have." (Regrettably becoming common)
B: "How about Jack? Is he an only exceptional person?"