Hard data on all the good fans:
XS Fan Tests
No need to go off anecdotal observations, everything you need to know is right there. You are interested in the highest CFM point of the 'Low Speed' fans, which corresponds to 12V (no rheobus).
Looks like your best bets are the Yate Loons from Petra's. The Scythe 'Minebea', Silverstone FN121, and Noctua 1200 are all right there too.
Personally, I run undervolted San Ace 1011s and Panaflo Hs. I hate the failure mode of sleeve bearing fans (grinding noise).