This question is probably the most controversial question when it comes to railroading.
Which 4-8-4 is the best?
SP GS-4
UP FEF-3
NYC Niagara
N&W J Class
Once again a "best" question cannot be answered without the further clarification of best in what regard. Clearly the argument of ones running today is a red herring because that much more a factor of where the money and interest and raw materials are. I am certain many of the BEST Northerns never had a chance to be preserved. Scrap metal is very valuable.
Best pulling, best looking (entirely subjective), most fuel efficient, best speed deamon, most reliable, most economical (least cost of ownership), etc.???
It is sort of like the Big Boy. It is "biggest" only if one is counting drawbar to drawbar length. It is not the heaviest, most powerful, biggest wheel arrangement, tallest, or even widest. It is only the biggest in one dimension.
Of the ones in your list I would have to submit the N&W J, as it is the most modern and evolved of the lot.
I would also contend that the "best" Northern isn't even in that list. Santa Fe 2900 class never got a chance to prove themselves because of Santa Fe's hurry to dieselize. The whole class all went to very early graves. No one has mentioned the RF&P (oops I see someone did!), Great Northern, Rock Islands, GTW, CN all magnificent machines and many deserving of being considered best in some way or another. I do strongly contend that the Northerns were the over all finest class of locomotive to grace the American rails regardless of which Road or Class. The Hudsons (favored by the CP over the Northerns for some reason) are a strong 2nd.
ASIDE - Sorry, I totally disagree (and Santa Fe management agrees with me) the Santa Fe 3700 class is in no way shape or form the "best" Northern.