The phrase 'ultimate question' is correct and usable in written English. You can use it when referring to the most essential or important question that needs to be asked on a particular matter. For example: The ultimate question is whether the proposed project is worth the cost.
So here's my ultimate question.
But the ultimate question remained.
Yet our proposals sidestepped the ultimate question.
But the ultimate question, of relevance to humans, remains unanswered.
Reichheld and Markey, "The Ultimate Question 2.0," 231.
This is the ultimate question in the privacy tradeoff.
The ultimate question: How will we pay for it?
The ultimate question is why we enjoy certain meals.
Yet the ultimate question remains: which player is better?
The ultimate question is whether adaptation alone is enough.
"But the ultimate question is, does this change outcomes for patients?
Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.
Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com