Jetfire

Jetfire (in Japanese: Skyfire in TF Super Link; Dreadrock in TF Galaxy Force) is probably the best second-in-command Optimus Prime could conceivably have. Jetfire is more than a yes-man; he's the crucial link between Prime's role-necessitated distance and his troops. Though professional and businesslike, Jetfire infuses his interactions with a down-to-earth friendliness. And where Optimus Prime (Convoy in Japanese version) is cautious, Jetfire is more prone to tempered bursts of gusto and bravado. While Optimus Prime is your leader who's your leader, Jetfire is your leader who's your friend. Because of this, Jetfire is very popular among the Autobots.

Most importantly, Jetfire is one of the only Autobots (Cybertrons in Japanese version) who will tell Optimus Prime to his face when he thinks he's wrong. It is this trait that makes Jetfire indispensable.

(Source: Transformers Wiki, edited)