80mm Hiper fan. The quietest fans I've used and with a fan controller they're about the only fans that I'd let people get away with calling 'silent'. No fan is truly silent (obviously) but these fans get pretty damn close. On a fan controller they'll be quieter than anything else in your PC without a shadow of doubt.
If you're looking for bigger, then the 120mm version would be a good bet, but I can't personally vouch for it. I'd actually bet money on it being good though.
Also, as a side note, from experience I've found that 80mm fans are always the quietest. They are the optimum in size to noise ratio including factors such as blade weight and air turbulance. Generally speaking it is better to have 2 quiet 80mm fans rather than 1 120mm fan. They won't rumble. Though if you're mounting on a heatsink then that may demand the 120mm size.