Actually from a game mechanics point of view fire is the best element. The Disintegrate perk causes your shock magic to destroy potential zombies, Deep Freeze causes frost magic to paralyze the target and there is bug that caused zombies to just walk in place and not do anything else if they were created from a body that was paralyzed. Fire just causes thing to run away briefly before they die. So, while not the best thematic choice, fire is the best technical choice.
Actually from a game mechanics point of view fire is the best element. The Disintegrate perk causes your shock magic to destroy potential zombies, Deep Freeze causes frost magic to paralyze the target and there is bug that caused zombies to just walk in place and not do anything else if they were created from a body that was paralyzed. Fire just causes thing to run away briefly before they die. So, while not the best thematic choice, fire is the best technical choice.