db is not a good indicator of fan quality, CFM is. If you have 2 fans, and fan A runs at about 500rpms slower than Fan B. But Fan A's CFM is 50% greater, then Fan A would be able to move more air than Fan B.
Dbam is a measure of loudness so lower is better cause its quieter, but Noobie is right about cfm and rpm's being your deciding factors,
Being a trade-off as things often are, between noise and cooling capability in this case,
You want to have the biggest amount of air moving through your case but at a volume you find acceptable, if a small fan is moving at a high speed, it will be loud and move X (Cfm) amount of air a minute) but a larger diameter fan can move the same amount of air at lower rpm's, thereby reducing noise,
I hope this helps you understand a bit more about fans but as for which is 'best'?
Same as everything else your going to find one person advocates his favourite brand, and someone else sells their idea to you, I use cheap ones but if you have money and want to spend 20 Dollars/Pounds on a fan, feel free
The generated noise is a VERY important aspect when you want to buy fans. The CFM ( it is the amount of air a fan moves) measures the performance of the fan, but maybe a high cfm fan is not the best option for you. For example, the DELTA AFB1212SHE is an impressive fan which has 152 CFM, but, of course, that comes with A LOT of noise: 53 dBA (it's is a jet engine).
When you buy a fan you must look for a good relationship between air flow and noise.
Here you have a very good articles that can help you choosing some good fans:
Anyway, I write you a short list of interesting fans (in my opinion):
Scythe "KAMA PWM" Fan - 92 x 92 x 25 mm - 2500rpm - 31.07dBA - 55.6CFM
Scythe "KAZE JYU" FAN - 100 x 100 x 25 mm - 2000rpm - 30.68dBA - 58.0CFM
Scythe "ULTRA KAZE" Fan - 120 x 120 x 38 mm - 3000rpm - 45.90dBA - 133.60CFM
Scythe "KAZE MARU" Fan - 140 x 140 x 25 mm -1900rpm - 41.13dBA - 104.54CFM
Scythe "GentleTyphoon" Fan - 120x120x25 mm - 1850 RPM - 28,0 dB(A) - 57,68CFM
Scythe "Slip Stream" 120 Fan - 120x120x25 mm - 1900rpm - 37,0 dB(A) - 110,31CFM
NOCTUA NF-R8 80x80x25 mm 1800 RPM 17,0 dB(A) 31,19CFM
NOCTUA NF-B9 92x92x25 mm 1600 RPM 17,6 dB(A) 37,85CFM
NOCTUA NF-P12 120x120x25 mm 1300 RPM 19,8 dB(A) 54,33CFM
NOCTUA NF-P14 140x140x25 mm 1200 RPM 19,6 dB(A) 64,92CFM
(The Noctua fans are an excellent choice if your priority is the quietness)
SILVERSTONE FM92 92x92x25 mm 3800RPM 48,6 dB(A) 80,92CFM
SILVERSTONE FM121 120x120x25 mm 2400RPM 39,5 dB(A) 110,03CFM (the FM123 is a very good option too)
Delta FFB 1212 EHE 120x120x38 MM 4000RPM 59 dB(A) 190CFM
Yate Loon D12SH-12 120x120x25 mm 2200RPM 40 dB(A) 88CFM
Panasonic Panaflo FBA12G 12U1C 120 x 120 x 38 mm - 2750RPM - 45,5db - 114,7CFM
Mechatronics MD1238X12B -120 x 120 x 38 mm -4500RPM - 62db - 205CFM
Keep in mind also that you need not (and probably should not, for longevity) run a fan at its full stated RPM. BIOS control, or a fan controller, can lower the speed of a fan. It will move less air, but also make a lot less noise. For example, I have a Scythe Ultra Kaze in my case, but it is nowhere near 45 dbA because it is running at a fairly low speed.