The best drive on the market (according to StorageReview.com and popular opinion) is IBM Deskstar series. The Deskstar series has a maximum sustained transfer rate of 37.5MB/Sec this barely justifies the upgrade to ATA66 for the extra 4.5MB/Sec sustained.
Some argue that ATA66 and ATA100 allow you to take further advantage of your harddrives "Burst Transfer Rate" however BTR only comes in to play when the drive transfers to and from it's own memory cache.
Thorin