If you want a good balance between noise and airflow the Scythe SFlex 1200rpm fans are excellent and well thought of on silent pc sites. OcUK sell them. Other good fans are the Nexus 120mm orange or black/white fans, but they push considerably less airflow. The YateLoons are the same fan but run at the full 12v, so would be a better bet. Sharkoon are also good but again less airflow. The Scythe are the best I have come across.
Lots of people recomemnd SilentX but they exaggerate about their db and airflow rating. Quite a few people on these forums also recommend the Akasa Ambers which I don't like. Rebadged YS-Techs. Noisy unless you volt mod them, and then they make a clicking noise. I also find the same to be true of the Acoutifan Dustproof 120mm models.
So in my opinion for a fan running at 12v in order of cooling capability with good noise profile.
Scythe SFlex 1200rpm
Yate Loon
Nexus
Sharkoon
Scythe Sflex 800rpm