Those that claim competency in building AR variant rifles generally recommend against building an AR10- at least for someone not experienced. There is no "mil spec" in the same sense that there is for ar15 rifles. Armalite vs DPMS/SR25 pattern, etc.
As to ranking, I'm sure someone will be along shortly to say that Knights Armament makes the only large frame ar variant rifle worth owning, and to "go be poor somewhere else..." In the US we (most of us anyway) have the great fortune that if a rifle- or some part of a rifle- is not up to snuff, we can easily change it. My poor understanding of NZ laws suggests you may be "married" to your purchase- at least relatively speaking. If you can swing it, I don't think anyone would have buyers remorse with a JP in their safe, or a SR25 for that matter.
There are a lot of factors to consider and "best" is so subjective as to not be a useful superlative.
Listed in no particular order...
Cost
Weight
Length
Ergonomics
Accuracy
Warranty
Factory suppprt
Reliability
Durability
"Hey look at my cool rifle" factor
How much will it cost to replace the OEM stuff that the rifle comes with but you don't like/want?
Hopefully you can get 20 people to give you a top 10. My guess is that 80% will be identical between all of them, with the only difference in the top 5 being the relative placement in that top 5.
Sorry that I can't be more help. I've only got a DPMS lr308 in my safe, and it won't make a top10 on anyone's list that has had these rifles flow through their fingers like beach sand. It does, however "punch well above its weight class" and would surprise the shit out of some gear snobs.