All around , Gentle Typhoons(AP-14, AP-15) for 120mm DC (non-PWM) ... unless you're going to do wiring for $30+ Sanyo Denki
for free air (no filters/obstructions) then you have other options such as Be Quiet Silent Wings 2 (close to max RPM, at low RPMs it is not that great). PWM: NF-P12 PWM is also a good choice, dB wise it is louder than Gentle Typhoons but generally it does not have harmonics or any PWM click.
If you size up to 140mm you have options like the Thermalright TY-140 series (120mm holes) , Phanteks 140mm fans (120mm holes), Noctua NF-A14 / NF-A15 PWM. doyll will probably drop by and tell you about how good TY-140 series is, they also happen to be cheap in the UK (Scan.co.uk) and Europe.
If money is an issue and you live in the USA there's some cheaper fans like the Arctic F12 / F14 (the PWM version may click though and the F14 doesn't seem as good as the F12 is compared to other fans of the size) that have so-so quality control (luck of the draw) and short cables with no accessories. They also have brittle blades so if you accidentally stop them they may break.
Corsair has their AF120 /AF140 (airflow) which is okay for exhaust but at ~$13 and $18 each respectively they are overpriced given the quality control. You do have Corsair's great service to fall back on but once the warranty period is up, good luck. Reviews suggest it has mechanical noise from the motor so there's still work to be done.
I combined the DC and PWM 120mm for comparison (since HARDWARE.FR failed to make a radiator chart I had to take matters into my own hards
^ rad was Hardware Labs SR1 120mm
* I left out Cougar, sorry. I should edit my graphs, but they're OK. They have a bit of mechanical noise at low RPMs.
* Arctic F12 CO is ball bearing like Gentle Typhoons
Gentle Typhoons appear to maintain more than 80% of flow even with a thick rad (Corsair SP120s too but they didn't have THAT much flow to begin with). Noctua NF-P12 PWM and Be Quiet Silent Wings 2 appears to maintain ~75% of flow.
Only caveat dB-wise with some fans is harmonics (Gentle Typhoon sometimes) and PWM click (low duty cycle for some PWM fans)AVAILABILITY
Superbiiz has AP-14 and AP-15
and Amazon http://www.amazon.com/Scythe-Gentle-Typhoon-1450rpm-45-9cfm/dp/B001Q6X0S6
and frozencpu has AP-15 http://www.frozencpu.com/products/10323/
edit: with Cougars:
and Judge AP-15 sound for yourself http://www.youtube.com/watch?v=a43MUaY9Xc4
A lot more videos over on Coolingtechnique too http://www.youtube.com/user/CoolingTechnique/videos
140mm fans http://www.hardware.fr/articles/886-26/recapitulatif-db-a-vs-cfm.html
There's also sound clips for 140mm fans over on hardwaremax http://www.hardwaremax.net/reviews/luefter/592-test-noctua-nf-a14-und-nf-a15-140-mm-luefter.html?showall=&start=5
and 120mm http://www.hardwaremax.net/reviews/luefter/467-test-scythe-gentletyphoon-und-thermalright-x-silent-120-mm-luefter.html?showall=&start=6Edited by AlphaC - 11/2/13 at 12:22pm