While the Kuuga Growing and Kuuga Mighty Form transformations are really damn good, I feel the first transformation for Agito deserves a nod too.
We, as the audience, know goofy and silly Shouichi is the one transforming and yet the scene is still able to effectively draw the viewer into the awe and surprise of G3 and the Unknown as this mysterious man, his features hidden behind the wall of light generated by his belt, slowly steps onto the field of battle for the first time.