The best replacement for dielectric is insulator.

The best alternatives to dielectric are rated as following:
★★★★★  insulator
★★★  insulation resistance
★  semiconductor
★  capacitance
★  silicone grease

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Speaking about dielectric, one probably shouldn't forget about capacitor.

Other search terms related to dielectric and their ratings are:
★★★★★ capacitor
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dielectric
insulator
insulation resistance