Water

Water is a chemical so rare even its existence is suspect on Cybertron, or at least so claims Ratbat.

(Note: Ratbat's likely confused, as several humans later managed to exist without environmental protection gear on Cybertron without drying up like old corn. Also, several characters, like Broadside, Seawatch, and several Pretenders would have entirely useless alternate forms without it.)

From Teletraan I: The Transformers Wiki, a Wikia wiki.