This is a very subjective!
Back in the '70's I would have said the Revell Funny cars were the very best scale kits EVER! (I built and scratchbuilt from more than 10 of those wonderful kits. Most of them the Vega funny.)
But.... (very big but), if someone never builds drag cars..... they wouldn't understand.
The Galaxie Chev kits could probably be put in the same category as the earlier Revell kits... but if one only desires to build Ford or Chrysler products....
And what about street rods? If a builder only likes building street rods then the viewpoint narrows to a handful of kits.
I have heard that the Revell 1957 "Black Widow" kit is one of the nicest scaled kits... but I don't like or build tri-five Chevrolets.
I had heard that the Fujimi 911 Porsche kits are one of the nicest scaled kits of all time back in 1989.... but I don't care for Porsche kits.
You see what I mean? It is subjective based on what subject matter the builder is interested in building and therefore the question needs more focus.