Samurai Sentai Shinkenger: The Light Samurai's Surprise Transformation (2009)

This can be viewed after Act 24 and their movie.

Kamen Rider Agito Special: A New Transformation (2001)

This was aired between episodes 35 and 36 and you’ll want to watch it then, too, as this conveys all of the emotional significance behind Agito’s final form. The debut of it in the show itself (in episode 37) assumes you’ve seen this as there’s not much in the way of specific explanation. It's worth noting that G3 Mild appears here.

Tokumei Sentai Go-Busters vs Beet Buster vs J (2012)