Hardware.info compared a whopping 103 120mm fans, you can check it out over here. The site compared the fans in terms of best temperatures, lowest noise levels, highest efficiency and price.
The humble case fan may very well be the simplest component of your PC: the component's sole purpose is to blow air from one side to the other, and preferably as much air as possible while producing as little noise as possible. Because it is essentially only these two aspects that matter, one might conclude that choosing the right case fan would be fairly straightforward. Unfortunately, things aren't quite as simple in practice. First, manufacturers will often times engage in creative accounting: the specified values for the amount of air that can be moved (the airflow) and the noise production will typically be determined in a rather creative fashion. Second, a case fan that under normal circumstances would offer a lot of airflow might disappoint after being attached to a radiator or a CPU heat sink, a phenomenon that has to do with static pressure. Third, different case fans may use different types of bearings, with lower quality bearings having a negative influence on the fan's life expectancy. And fourth, when using a high-end PC case with a window, the looks of the case fans are important as well.