Hmm, tricky.
The newest Seagate's are very good - new technology (perpendicular recording, so faster than all other 7200rpm drives), 5 yr warranty as standard, they are reasonably quiet however this is subjective and many say theirs is loud (which could be a result of an apparent loud batch which has now been resolved).
Samsung Spinpoints are meant to be quiet (the quietest) but I just don't trust the company enough yet - they haven't been in the HDU business as long as many of their rivals and I'm yet to be convinced.
WD's are pretty quiet and seem a good all-rounder in terms of this but often lack NCQ (native command queuing - useful when multiple things access the disk at the same time) and the fact that no drive can keep up with the Seagate 7200.10's at the moment.
You could be safe and get a Silentmaxx enclosure for the drive... but in fairness, my 320GB .10 is very quiet.
