Is tonearm cable the most important cable in the system ?

There is no "most important cable" in anyone's system.  Every cable in your system contributes to the resultant sound.  If you doubt that, take a quality cable you currently use and replace it with the stock throw- away cables that come free in the box.  That I do believe will settle the matter.

The cable is much less critical if you can run it balanced into a balanced, differential phono preamplifier.

@hifiman5 Good point.  Your cables should be treated as a component itself.

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.

Leeds - I wonder.... where exactly would you get a seperate ground reference for left and right channels in order to run a "truly balanced" XLR cable from a phono cartridge that has only 4 pins (left/right +/- )????  Stereo Phono cartridges are INHERENTLY balanced devices no matter what connector you happen to be using at the phono stage.