Of course water is the best medium for cooling, but its conductive, I have heard that distilled water has no metals, but there is another water called heavy water that is put to use in nuclear heat exchangers, and even used in coil designs for metal detectors.
But those are impractical
I guess you could do a search on best heat transference, I would think maybe motor oil. just because of its properties for heat in auto engines. Also mineral oil.
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