which of the N models is the most collectable?
Don't have a good answer for you. I can say that I bought a Jubilee because it has more hp than an N, live hydraulics and overhead valves.
My next Ford will have live PTO. I don't collect tractors to make money, I collect old tractors to use.
I don't think too many people will get rich collecting old tractors. It's something I grew up with. At my age, tractor theorpy is the best thing.