Plain and simple, who's interpretation of these two characters do you honestly like best? Don't rely on just nostalgia, but also think, who's voice do you truly believe (to this date) has made a good reflection of the overall idea of the character? Personally, my favorite Optimus belongs to Garry Chalk, I really like how he sounds. I grew up and saw some G1 reruns as a wee lad, but it never strongly impacted me, but I was around for Beast Wars/Machines, and Primal really landed one for me. I also hear his voice in those Cybertron cartoons (which I didn't watch/care for) but it just sounded like a perfect fit. It has that flair of heroism, integrity, and, at times, naivety. As for Megatron, while I did enjoy Hugo Weaving with the monster-ish voice, to help give a menasing and fearful essence to Megatron. Though, I had to really consider that these are basically supposed to be people, mortals, with giant metallic bodies, and simply having a monster as the "bad guy" really doesn't do it for me when I try to shift the characters out of their robot bodies and into that of fleshlings. So then when I continue to think, one voice that really REALLY made sense to me was Corey Burton's in Animated. He sounds collected and confident, how a military leader should sound. Not reacting to every emotional impulse, but knowing when to layeth the smacketh down, when necessary.