For all I know, Ammonia is actually the best, it is just explosive, extremely corrosive and can cause health issues. We can only legally use it for commercial applications.
The scroll type of compressors are the best and have good longevity for heat pumps, since the patent ran out, they are much cheaper now.
You probably know this but need to change the lubricant oil as well as a few other things to check.
This describes going the other way… but you would be checking the same things.