Sengoku Basara Ni: Ryuko, Itadaki no Chikai! Atsuki Mirai e Kakeru Tamashii!!

I can understand why this special was kept separate from the rest of the anime. It's humorous nature didn't really fit the feel that the end of the first season held. But nonetheless, it's very good and any fan of the anime would love it.

It highlights the relationship between Yukimura and his retainers and shows how Shingen and Sasuke see him. This special is good for a great laugh if you're a fan of the passionate rivalry between Masamune and Yukimura, Sasuke's ninja antics, or the over-the-top-ness of that comes with anything associated with Sengoku Basara.