If you include the tonearm, I would say its internal wire. Especially if you eliminate all the junctures, solder joints, and connectors, creating an uninterrupted run of wire from the cartridge pins to the RCA or XLR plugs. The reason being the output voltage of cartridges is the lowest in the entire system chain.
Where do cables upgrades have the most impact?
Assuming all existing cables in a system are atleast mid-grade (not junk), which cables/interconnects should be upgrade first and in what order? Where should one start?