If silence is your priority, I don't think Noctua's fans are easily beaten. The trouble with fans is there really isn't a 'best' one, it's all a compromise between performance, noise, cost and aesthetic appeal. Not to mention the difference between higher airflow and higher static pressure etc! Having said all that, this case hasn't really been designed with super-silent gaming in mind - it's really about the airflow. There are lots of cut-outs where noise leaks through, for example.
Personally I like the Scythe Gentle Typhoon AP-15s. I originally bought 2 after reading many reviews, and this one springs to mind: http://martinsliquidlab.org/2013/05/07/fan-testing-round-12/
. You can see the AP-15 (at 12v) achieving air velocity of ~600FPM at only 46dBA.
However, I was looking for a higher CFM rating for the 3 front intake fans, as I felt the ~60CFM on the GTs could be beaten easily. After a lot of research, it turns out that in real world applications the max figures quoted for both CFM and static pressure are really only a guide. Often they're not accurate, and even when the are they represent a point on the PQ curve that doesn't really hold any relevance for normal use. Seeing as the front intakes pull through both a dust filter and a mesh filter behind that, the higher static pressure of something like a GT actually comes in really useful. They also tend to pump out more CFM along the PQ curve than any comparable fan I've seen (it has a 'bump' on the curve that is right where real-world use comes into play) so I decided to get another 3 for the intake.
Check this out:
The compromise? Well, they're not quite as quiet as Noctuas, from what I can tell. But I game either with headphones or speakers up reasonably loud, so this isn't a dealbreaker for me. Second, while the fan looks quite nice (imho at least!) the cables aren't braided - which is inexcusable at the price point these come at - so you either need to have some clever cable routing going on or deal with multicoloured wires spoiling your case's aesthetics!