What you need is determined by what you're cooling(cpu/graphics card, etc), and how much noise you really want.
The absolute best 80mm quiet, but high output fans would probably be the Sanyo-Denki's and Papbst fans(both are fairly hard to find and can be expensive).
The best value 80mm quiet/low output is the Panaflo 80mm L1A, available from [l]www.teamawe.com[/l].
YOu could also go with the Adda 80mm quiet fan, or the PcPowerCooling Silencer(even though the silencer's actual performance is a bit weaker than the 80mm Panaflo L1A, it is a bit quieter).
Mike