For me this one is a no brainer. Starscream. Just looking at my voyager from the first film next to the leader class (hell even the voyager) from the second is such a huge difference. Even the deluxe from the newest film is a marked improvement. Now don't get me wrong, I actually do like the original voyager mold, got some good memories of it too (mostly when I first got it, was right before going to Six Flags with my uncle), but damn is the difference in design and quality noticeable. Just crazy what a few years of engineering can do. The original leader Prime compared to the RotF leader is another pretty drastic change, though the original leader is still pretty damn solid. And Megatron..well it's harder to really judge with him since he's changed forms across each film. I wouldn't be too surprised to see Bumblebee get a mention here, but I just don't feel the changes and improvements from the original concept camaro mold are noteworthy enough. Ironhide maybe.