SubZero manufactured the first professional refrigerator back in the 50s. With the building boom in the last 20 years (deduct the last 2.5), every manufacturer produced these ultra high end refrigerators.
I have always defined a professional refrigerator simply as products with compressors on the top like a commercial refrigerator.
Here is what they look like and what to look for...
Thermador (Bosch, Gaggenau, Miele have the same box with different interior)
The best of the bunch is still SubZero with 2 compressors, the best seal and temperature control. In 2009, they added an air scrubber to keep food fresher and an advanced water purifier as well. However, there are less expensive alternatives...Here are the best of the rest.
Bosch, Thermador, Miele, Gaggenau: This is an integrated refrigerator, which is really great looking with panels as it blends perfectly.
Liebherr: This German company is also the greenest and has some interesting configurations of products.
Viking: Bought Amana's tooling 10 years ago, and has finally righted the ship
1. Rebates: Even the venerable SubZero company has one when combined with their sister company Wolf...There is not a huge price spread between all of them and features have been replicated.
2. Panel Vs Stainless: Look at the product with both.
3. Repair: Insure there is service available....it is different than fixing a regular fridge.
4. Price shock? Consider a regular shallow (non top compressor) refrigerator and save $5,000 plus.