I use Yate Loon Fans:http://www.svc.com/d12sl-12-or.html
It states 28dBA, but it is quieter than that.
Nexus fans are basically cherry-picked Yate Loons that spins at lower RPMs and are quiet:http://www.svc.com/d12sl-12-bw.html
Scythe Fans are pretty quiet as well, I believe I have the following one (I don't really feel like taking my PC apart):http://www.svc.com/sy-1225sl12l.html
Lastly, I recommend the following Noctua Fan; Noctua has developed a reputation for quiet fans with their unique designs:http://www.svc.com/nf-s12-800.html