In my opinion, a good parang is one that has the heft and length to perform an outstanding job. Of course, this would depend on what you use it for.
It can be sharpened quite easily. Holds an edge for a long time. It will be susceptible to rust but shouldn't be a problem with a little help from oil.
It is almost maintenance free. It is cheap compared to other blades. If you lose it, it wouldn't cost you an arm and a leg to replace it.
Usually, you would want to have at least two parang in your house, just in case, you have to replace the other one immediately for a good reason.
Parang is common in Southeast Asia. Almost every household has one. It is used for chopping wood, trimming arm-sized branches to killing snakes if necessary.
A good parang is not known for its good looks. Its finishing is slightly crude or rough, unless you get one done by an excellent knifemaker.
All parang are meant to be used, and used regularly.
Basically, it is cheap, tough and lasts a long time, and easily replaceable.