1/16 - great if you like vintage tractors in isolation, but what if you like farms? Or combines? What happens when vintage tractors get "newer" and therefore larger? does one pack in the 1/16?
Does it matter that a 1/32 tractor lacks certain detail when it would be too small to be of major relavence? It depends on how anally you admire your tractors, whether you study the engine block or sit back and look at a tractor pulling an implement in the field.
There have been some terrible generalizations in this thread which makes it painful to read. You could say why make a tractor in 1/16, when a tractor in 1/12 will be so much more detailed, or why go for 1/32 when 1/64 will take up less space, the answer, there is no best scale for anything