Is tonearm cable the most important cable in the system ?

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.

25 yrs ago I let a pro rewire my SME III w/ Van den Hul MCS-300 pure silver path from cart pins to phono inputs. It made a HUGE improvement to the already superb arm (designed especially to HC carts), dynamics and details/nuances and the high registerd in particular got much refined and not showing even a hint of harshness. But I must point out that silver wire instantly reveals faults such as sibilance w/ your carts or your records. Then I realized that the extremely low voltage output from a cartridge needs and deserves nothing but the best quality wires available... I haven ´t looked back since.

I also use Trans-Fi audio´s linear airbearing arm w/ uninterrupted silver path from cart pins to phono input (125 cm to be exact).

I bought a pair of the KLEI Absolute Harmony RCA’s (that I will send Vic at Trans-Fi soon) on the ends of the 1-piece silver loom with which he will wire my Terminator Arm aluminium wand. Regards and thanks bdp24 :)