Well, there are many points I can put forward to check while buying the best case fans. But as you said, you are particularly talking about the one which doesn't create a lot of noise or in other terms quietest case fan. So My choice would be Arctic F12 PWM PROS It has an anti-vibration mounting system. It has a very unique blade design which provides more cooling. Aesthetically well designed. CONS It uses a two fan system mechanism. It won't fit smaller case sizes. BE QUIET! BL070 SILENTWINGS 3 PROS It is perfect for small or medium size cases It is very quiet because of the fluid mechanism and bearing design It has 4 pin connector which in turn gives you more control options CONS None as of now, but to be honest My friend notices some little noise, but NOT ME both of them have topmost level performance and known for their quiet nature.