Samsung. Especially the drives with the Nidec FDB motors, the quietest drives on the market. Performance takes a little bit of a hit because of the softer seeks, but there is no question that Samsung SpinPoint P80 series is the quietest HDD you can buy (well, aside from the long discontinued 'Cuda ATA V). 3 year warranty on all of their drives is just gravy.
The PATA Seagate 'Cuda 7200.7s are pretty quiet too, but as the SilentPC Review article linked in the Buyer's Guide suggests, buying a 7200.7 for quiet operation is something of a crapshoot, as you may get a drive that has Automated Acoustics Management locked off.
Yeh, not all of the drives shipped with AAM locked off. Like I said, something of a crapshoot. I'm too lazy to look up if there was any rhyme or reason to which drives shipped locked, but I'm sure a search would turn up some clues.