Assassin's Creed: Brotherhood - Whoops...
I didn't remove my Helmschmied Drachen armour costume before starting the Cristina missions, so it didn't change to Ezio's Florentine Noble outfit... which meant 40-something year old Ezio was being a silly noob with a 17-year old's voice. Hilariously adorable.
The Helmschmied Drachen was available in selected Collector's Editions of Brotherhood (in my case, the Codex Edition) but it, and the other CE exclusives like the MP characters, are available with the Da Vinci Dissapearance DLC. Please stop asking.