Is tonearm cable the most important cable in the system ?

Yes of course every cable is important but not necessarilly equally important in each particular system. Often the closer to the source the more important. I am not talking about power cords now.
When upgrading, most of us cannot afford upgrading all the cabling at the same time, especially when doing serious upgrades.

williewonka, I don't think I will have this Nottingham arm modified. One reason - I might upgrade it to the newest Nott arm in the next couple of years, and another - I cannot be certain that it will actually sound better. I could just upgrade the RCA connectors, I guess.
So far, every interconnect upgrade from the Acoustech phono to Redgum RGi120 integrated yields big improvements, and I didn't start with junk, I started with old Purist Maximus. I guess, Nottingham wiring is quite good. So is $500 Goldring 1042 MM cartridge. I am impressed by both.

Syntax, your ancestors must've come from Ancient Greece.

No I have an original Nottingham Spacearm. I heard some people had the wiring replaced with the Discovery and something else - it was not better.