It's all very well that the shopping results on newegg tell me how loud the fan is in dB and how airflow it does in cfm. But what does that really mean, I (like everyone else I guess) need a quiet yet good air flow fan. Sure the lower dB the better and the higher the CFM the better, but it seems that if you want a quiet fan the air flow goes down and vice versa, so where's the middle spot?
The middle spot depends on your case and components. Different case designs need different levels of airflow to cool the same components. Hotter components (faster processors, more disks etc) need more airflow than cooler components (eg laptop processors and SSDs).
Significantly, the nominal dB and airflow ratings of fans often have little relation to how they behave once in a case. Generally if you're looking at fans in the 20-30 dB range then they'll be quiet enough for most people in an "office" type environment. You'll want quieter fans if the computer is going to sit in a living space. Bigger fans will also push more air (higher CFM) for the same level of noise than a smaller fan.
There is, in short, no "once size fits all" answer to what is actually a very complex question.
I would strongly recommend you read Silent PC Review, where they review many components and you'll get a better feel for what makes a good (and bad) fan (or other item).