Not only the best toy in the RiD line, but the best toy ever... Optimus Prime. Three toys in one, the figure was fun enough by itself with the normal robot and super robot but the ability to combine with Ultra Magnus too to form Omega Prime... The engineering that went into the two toys was impressive (although the transformation into Omega Prime was very complex), and for some reason, Optimus Prime transforming into a fire engine seemed so appropriate.
I believe the best thing about RiD was how it gave us some interesting combiners. Particularly Rail Racer, a three-component combiner was rare and the transformation to combine the three bots was interesting. The individual robots each looked good and were unique, transforming into bullet trains. Rail Racer is the second best combiner figure ever (behind Omega Prime).
Landfill was good, another odd one with only having four component robots. The gimmick that involved any of the three limb bots being able to be a leg or upper body was fun and cleverly engineered.
Megatron was also interesting with this multiple modes (although a few of them was a write-off), some creative ones such as the giant hand (when I first learned that the toy actually does that after watching the cartoon, it made me wanna run out and get it right away) and cooler ones such as the twin-headed dragon.
Overall though, all the new molds in the RiD line was spectacular.
"sometimes the things you see might not be real and the things that are real you might not see"