The Energon Pub

Thundercracker because of his doubt and Starscream since he was accepted into the autobot too easily.Jetfire was so happy that he change sides and no question ask even thought Starscream would be least one to change side. He probably tired of Megatron bullying, powerhogging, abuse,insults and blaming him for everything. He was also powerhungry and believe firmly in the decepticon cause and was the second in command next to
Megatron. He would be least of the decepticon who Megs would let change side.

ARMADA Starscream joined the Autobots. G1 and Armada are different continuities, as it's been pointed out. Thudnercrack, however, maybe, but he shows contempt for non-flying creatures. If Sunstreaker's a con because of his contempt for others, so's Thundercracker, and he's goin' nowhere.