I like the classic Optimus Prime because the transformation is straight forward but could be more skeletal with the vehicle outer parts as disguise. Same as Gundam (tho I don't watch one). RX-78 is my favourite. The recent Gundam designs seem a little too busy with generic head design and wings every where.
This is why I like Macross concept which Fighter (fast pursuit mode), GERWALK (hover and better aerial acrobatic mode) and Battroid which I believed built for melee combat with giant alien.
Michael Bay's Transformers would probably cause headache for toymakers. The recent ones even smoke, japanese and dinosaurs. How does that happen in Cybertron? They have female robot but yet does not serve purpose in reproduction. Too much strayed away from the original concept I think.