Best refrigerant depends on size of the unit as well as the temperature. If the system is small enough, I would use a DX and probably R507. I don't like to use the 400 series as they are all blends where 507 is an azetrope. If it is a cooler application, look at R-134A.

For larger plants, look at pumped recirculator systems on ammonia.

My 2 cents worth.

Ken