Sanicare Abri-Form M3. They're cloth-backed and very quiet. Tena Slip Maxi are also very quiet, as are Depend Briefs (which are the current mainstream Depend tape-on diaper line in Australia).
Of the medical-grade diapers, Molicare Super Plus were more noisy (the last time I tried them) due to their plastic coating, which makes an 'ssssss' sound if something slides across it - considering that the proper fit for most diapers has at least a little bit above where the waistband of the pants usually sits, this is something that happens a lot.
The noisiest diapers I've worn are Bambinos - every kind of Bambino crinkles with every step.
With plastic pants, the quietest I've worn are Leakmaster standard pull-on plastic pants, which make no appreciable noise.
The second quietest are Gary Manufacturing PVC snap-on pants, which don't make much noise when you're moving, but make noise if crinkled with the hand, or if you sit down the wrong way.
The third quietest are the polyurethane laminate (PUL) waterproof pants in Netti CombiAqua swim diapers - I've used them as emergency plastic pants before. They crinkle quietly with movement and the outer cover rustles when swiped across.
The least quiet, by a long shot, are Gary Manufacturing Euroflex pull-on pants. They crinkle loudly - loudly enough to be noticed at a distance by a bystander, albeit an AB/DL-aware one - with basically every movement of your body. The upside is that the material is quite strong and the cut liberal, so if discretion isn't a worry, these pants are good.