They cost a lot, but I haven't found better 80mm fans
anywhere. They're
Comair-Rotron's high end 80x25mm fan, the Flight II DC. Great airflow, nearly silent (feathered edge blades) and ZERO vibration while running...these beauties are IMO worth every penny. They weigh over 2x as much as a 80x25mm Sunon or Panaflo and are built to very close tolerances...you can barely see light between the impeller blades and the venturi. Far superior to even the vaunted Papst imports and equal in price.
If you don't feel like shelling out close to $100 for a set of 4 fans (i.e. you aren't obsessed with the best) then go with
Hoot's recommendation, which are excellent fans in their own right.
Don't get a Delta focused flow fan for case cooling...they are too thick for most cases' fan mounts (38mm vice 25mm) and therefore will not fit.