While Noctua fans are often hailed as one of the best, I really can't get over their color scheme looking like an aged appliance from the 80s, so I would personally suggest investing in some Cryorig fans:
http://www.cryorig.com/qf120_us.php
They come in 3 available types depending on your needs, the price is quite right for their quality and extras they come with, and they sport a nice 3 year warranty out of the box too. Having two of the silent ones in my current case, I'm very satisfied both with the airflow through 2 filtered meshes (panel to panel) and the hum they make at full RPM.
Hope this helps!