I just installed 3 Scythe Slip Stream PWMs into a Lian-Li PC-9F. This case allows for pretty good airflow, so I can run them at relatively low speed (~650 for your basic web browsing) and I can't hear them at all. About the only negative is that they have sleeve bearings, but they're not expensive either.
Cougar vortex (Easy to find in both PWM or normal) or Noctua nf-f12 are the 2 top ones from everything I have read about. I have 2x corsair AF120 QE's and I have heard mixed things about them. I'd test them for you right now, but I am still missing a mobo/cpu for my system
The corsairs are not PWM, but they have 2 modes you can pick from (using the silencer cord or not).
Just for some closure - I ended up buying a few of the Cougar Vortex 1200 revs (not PWM) and they have been great so far. Really like the build quality and, while they aren't completely silent, they are very quiet.