What brand/type of 120mm fans are the quietest and most effective?

[Apokalipse]

I don't know, but there are plenty of independent reviewers who do.
For example:
SPCR's Fan Roundup #4: 120mm Fans II | silentpcreview.com
Fan Roundup #5: Attack of the 120 Scythes | silentpcreview.com
MADSHRIMPS - Hardware Reviews ,Crazy Projects, Modding Tutorials and Overclocking
26 120mm Fans Tested and Compared - XtremeSystems Forums

also, if a fan is designed to have variable speed, the manufacturer might just list the noise level of low speed and the airflow of high speed.