Radio or ignition...
https://www.hotrod.com/articles/1006rc-ignition-wiring/
If link doesn't work. Think most will agree if not they can disagree.
First off, your particular ignition system will tell you what type of plug wire you should or-more importantly-shouldn’t use. Early points-style ignitions and magnetos used low-resistance wires to carry a lower voltage as compared to modern systems, which can easily carry up to twice the load, thus requiring a higher tolerance wire. Running a modern, spiral-core suppression wire with the early ignition can actually decrease the voltage being carried to the plugs; running the early solid-cores with a high-output (breakerless, capacitive discharge, or HEI) system can easily result in damaged electronic components for the exact opposite reason. There’s more to a plug wire than the color of its skin.
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