TFP Optimus Prime just seems perpetually tired and sad to me. This tiny group of Autobots is all he has left from his entire army. I get the sense that he's putting on a brave face, but secretly thinks it's hopeless. He's not that engaging, but his low-key presence actually makes him one of the more tolerable Optimi to me. (TFA Optimus is my favorite however. Making him a rookie was brilliant.) What annoys me about most Optimi is the way the other Autobots hero-worship him and that "father knows best" air he usually has. In TFP the other Autobots admire him, sure, but he's not so much on a pedestal and they're perfectly willing to argue with him when they disagree.