Cartoon-wise, my spontaneous answer is "Cyclonus." I'm basing this on his great loyalty to Galvatron, which was evident throughout, but esp. in FFoD (Cyclonus didn't rest until he had brought Galvatron back) and in Webworld (Cyclonus tried to find a cure for his leader instead of disposing of him). Also, in The Killing Jar, Cyclonus saved Ultra Magnus' life.