For metal cables, in old versions of IC2 and/or IC2 Classic:
* Tin is best for ultra-low voltage (which is only encountered when you're connecting a solar/wind/water farm to a storage block).
* Copper is best for low voltage (outputted by batboxes).
* Gold is best for medium voltage (outputted by MFEs).
* Iron is best for high voltage (outputted by MFSUs).
* Iron is best for extreme voltage (outputted by HV transformers).
Glass fibre is equivalent to tin for ultra-low voltage, and better for every other voltage, however it's expensive.
Note: Gold cable+MV is better than copper cable+LV, because the increase in voltage more than compensates for the increase in loss-per-packet. This also means iron cable+HV or iron cable+EV is better than gold cable+MV.
(TLDR Higher-tier cable is better, but only if used in conjunction with higher voltages! This is almost always misunderstood!)