Based on my research, when you take into account jackfruit fruit quality and tree quality, I've come to the conclusion that 'J-31' comes out on top.
I am excluding from this list 'seedling jackfruits,' unless they are available for sale/propagation.
It's common knowledge that 'Black Gold' is known to be a great jackfruit tree, with sweet fruit. The only drawback is that its fruit is soft and not crunchy. Crunchy jackfruit fruit is very desired by the Asian community, while the soft flesh kind is not. Because of this, I have to agree with you that perhaps it probably shouldn't be at number two on my list. Although this may be true, I can't wait to taste the soft flesh of 'Black Gold,' when it starts to bear fruit.
The NS-1 has been reported to taste a bit better than the J-31, but the NS-1 is reported to have a problem(s) with the tree. Jeff has reported it to look 'scraggly' at a certain time(s) of the year.
From what I've been able to gather: (1) the best jackfruit 'cultivar' taste and quality fruit is the 'NS-1,' (2) the best quality jackfruit tree, is the 'Black Gold.' And, (3) the jackfruit cultivar with 'both' the best fruit quality/taste & best quality tree, is the 'J-31.'
Ah ha.....OK, so this was like a research thesis. OK....well, I find it generally well reasoned but I am not sure that you would cone to the same conclusions of you had all of the world's best jakfruit trees and fruit in front of you for actual inspection and tasting......or even the other cultivars available here that you have not yet tried.
A couple of comments about your choices:
From my experience, Black Gold is a great tree as far as its look and growth habit. I haven't seen many others that can rival it full canopy and excellent disease resistance.....but I am sure there are some somewhere that could. However, maybe not locally at this time anyway.
NS-1 is an excellent tasting jakfruit but not the best that I have ever tried and I am sure there are better ones than the ones that I have tried out there in the world yet to be tasted by me. Even locally, NS1 was surpassed in flavor for me even by J-31 . Of course, this is a matter of opinion.
Sweet Fairchild, I have yet to taste and therefore cannot comment.
J-31 is a good producer and an excellent fruit taste-wise. Whether it is the best considering the tree and fruit quality its highly debatable.
There are a few other jaks I think you should try before you reach your final conclusions. I know you are just talking about commercially available, in the area, trees. But there are quite a few to still consider. You have the Mai group......1,2,and 3.....of which probably 1 and 3 might be considerations. Then there are the Bangkok Lemon, Lemon Crunch and red fleshed fruits, like Borneo Red and Dang Suria. I haven't had the latter as of yet either, so I really can't give you too much in the way of guidance.
Then there are the Australian jaks that I am dying to try. I know since those are unavailable here they do not come into consideration.
So my point....keep an open mind (and mouth) and keep tasting and analyzing.....and of course posting.