For sheer power and volume, the 1288A is our loudest machine. This is available primarily through audiologists and has a decibel range up to 90 DB. It has in-line volume control, so the volume range starts at zero and goes up (obviously) from there.
Next in line are the portable (but powerful!) Rohm and Hushh units. These are readily available at retail, and they are small but mighty, ranging from zero to 85 dB.
The Dohm products - all of the products which feature a fan inside - have a volume range between 50-75 dB. This masks the majority of the quiet but annoying noises that disrupt our sleep, as well as some louder, more obnoxious tones.
If you'd like a little more information, here's a quick product comparison.