When considering a case fan there are multiple factors involved, such as price, life expectancy, connection style, voltage, RPM etc.... but this thread is the best CFM/dB ratio, so I've done some math and in terms of CFM/dBA here is a list of each fans performance:
Manufacturer Model........................................CFM/dBA Cubic Feet of Air Moved per Minute per Decibel
Scythe S-FLEX SFF21D....................................33.5/8.7 = 3.851
Delta 120x38mm TFB1212GHE......................220.01/65 = 3.385
Delta 120x38mm FFB1212EHE...........................190/59 = 3.220
Delta 120x38mm FFB1212VHE......................151.85/53 = 2.865
Silverstone FM-121........................................110/39.5 = 2.785
Delta 120x38mm Triple Blade High Speed...141.96/52.5 = 2.686
Panaflo H1.................................................103.8/41.5 = 2.501
Akasa AK-183-L2B..........................................44.8/18 = 2.489
Panaflo M1.....................................................86.5/35 = 2.471
Scythe S-FLEX SFF21E.....................................49/20.1 = 2.438
Arctic Fan 12................................................56.3/23.5 = 2.396
Panaflo L1.......................................................68.9/30 = 2.296
Scythe S-FLEX SFF21F......................................63.7/28 = 2.275
Dynatron DF1212BA..................................93.08/41.39 = 2.249
Delta 120x38mm Triple Blade Low Speed.........75.93/34 = 2.233
PAPST 4412 FGLL................................................40/18 = 2.222
Akasa AK-174BK-S........................................38.4/17.5 = 2.194
Evercool Aluminum FAN-AL 12025.........................79/36 = 2.194
Yate Loon D12SM-12.........................................70.5/33 = 2.164
Scythe DFS122512L....................................60.09/27.92 = 2.152
Scythe DFB122512LS..................................45.07/20.94 = 2.152
Scythe DFS122512LS..................................45.07/20.94 = 2.152
Akasa AK-174BK........................................59.05/29.75 = 1.985
Cooler Master SUF-S12-EG...............................42.73/22 = 1.942
Delta 120x25mm Triple Blade Low Speed...........57.21/31 = 1.845
PAPST 4312 MV....................................................39/22 = 1.773
Yate Loon D12SL-12..............................................47/28 = 1.678
Nexus D12SL-12............................................36.87/22.8 = 1.617
unfortunately some of the best performers are either way too loud or don't move enough air to be used in most home PCs, but I figured I'd list 'em anyway for the hell of it.
I tried to find the most respectable sources I could for the CFM and dBA ratings, but typos do exist on the web and sometimes manufacturers straight out lie, so if anyone sees anything that they dont think matches up to reality, please post a revised version, I wont be offended.