Noctua :)
Noctua NF-A4X20 PWN bought in 2 copies to ventilate my Nas in rack which was struggling with the +40°c daily.
Without using the accessories supplied with these fans, I fitted them instead of the original ones as they were...
The result is a lot more ventilation for a silence, which when they are full, is inferior to those of origin when they were at rest.
In short: Noctua is very good and the brown/beige colour is as gratifying on a NAS as Ferrari red on an Italian car.