Asking for quietest case fan is like asking for quietest car tire. One that is not moving is the simple answer for a simple question.
To determine what will be the quietest system requires choosing a case that will flow air as needed, and that depends on how much airflow components need. A system using a 120mm fan on CPU cooler and 2x 80mm fans on GPU cooler only needs about half as much airflow as a system using 140mm fan on CPU cooler and 3x 90mm fans on GPU cooler.
And even that answer is over simplified.
It also depends on what size the fans are. While the below listings are available in both 120mm and 140mm, there are some other are at only available in 120mm or 140mm.
Some of the very best fans available right now are be quiet! Silent Wings 3. Others are Phanteks PH-FxxxMP, Thermalright TY-147A SQ( only 140mm),