It is correct and usable in written English.
You can use it to describe the largest gap between two values or ideas. For example, "The greatest difference between the two cars was the fuel economy."
But the greatest difference is patience.
But the greatest difference is the passage of time.
But perhaps the greatest difference is the economy.
The greatest difference was in attitudes to food.
This spending often makes the greatest difference on election day.
"The greatest difference," Theroux writes when comparing the two journeys, "was in me.
But it has probably made the greatest difference to enthusiasts of lesser-known sports.
This is the greatest difference between the India of Indira's Emergency and the India of today.
Perhaps the greatest difference with heroin and opioids, caseworkers said, is their iron grasp.
This frees physicians to spend more time where their expertise makes the greatest difference.
Exotics had higher germination rates than natives with the greatest difference in mature forests.
