I have no experience with any brand other than National Luna. I can say that Paul May of Equipt Outfitters offered unequivacably the best customer service I have ever experienced.
Equipt is National Luna's sole US importer, and all sales are through them (other dealers are simply drop-shipped orders through Paul.) I purchased a National Luna 90Twin through a source that brought the fridge into the US directly from South Africa. When I had a problem, I did not have a US warranty, and that the South Africa warranty I did have required me to ship the unit to the factory for any covered repairs.
I contacted Paul for any suggestions on what to look for and what I could test, stating up front that I did not purchase from him, nor did I expect or ask for any warranty work through Equipt. Despite that, Paul spent quite a bit of time over several phone calls trying to get us back up and running. (We were five days into a six week trip when the fridge failed.) Paul even called and talked to the owner of National Luna in South Africa to try to get my unit covered under his US warranty service. When that didn't work Paul offered to buy my unit back for what I paid, and offered an incredibly substantial discount on the difference to buy a new unit from him with a US warranty. I just couldn't justify the additional cost, though.
In the end, Paul was unable to help. But he put in a valiant effort, despite me having never spent a dime at Equipt. Even worse, I had stopped in at Equipt the year before to look at the K9 rack, and I'd told him I'd order one when I got home. Then I changed my mind and bought the Prinsu rack. I owe them a big order at some point!!
Second place award goes to the place where I actually bought the 90 Twin fridge. After denying any responsibility for warranty work on the demo unit I bought, and offering no suggestions on what I could do, he eventually traded his own personal 90 Twin for my dead unit, and sent it back to South Africa with an existing shipment he was preparing.
New unit is working flawlessly, and everyone involved stated that they have either never or very rarely had any problems with National Luna.