Is tonearm cable the most important cable in the system ?

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

for me it's the cable between the phono SUT and preamp 

There's little you can do about the wiring to cartridge when the TT's wiring is "captive" (ie, non-detachable/replaceable). But if you have RCA L+R jacks on outside of your TT, what you CAN do is up your game by getting the best possible cable to connect TT's outputs to preamp. 

I won't try to recommend cable for that. There are many worthy candidates. I'll just say this:

1 - In doing this, you are increasing the sonic potential of your TT in the same manner as any other source component that connects to the TT via RCA cable

2 - The difference is that the signal coming out of the TT (assuming RIAA equalization is done in the preamp or phono amp, not in the TT) is even lower in level than the typical 2.0-3.0 volts being output by tape decks, CD players/DACs, etc. The TT output signal is more fragile than that--so will benefit (perhaps dramatically) by a really good RCA cable w/low capacitance and other qualities of robust design.

If you have a Nottingham with the Rega tonearm, you can send the tonearm to Britaudio (www.britaudio.com). I sent them my tonearm and had Mr. Wharton install the Incognito wiring harness. I now have better wiring and a proper ground. I sent out the tonearm by UPS on a Monday (from Ohio) and had it back by the weekend.

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.