It would have to be the last episode of season 8, as it was FULL of action and i would have loved to help Coop find Phoebe, well actually SAVE all the sisters.
Also i probably wouldn't mind the episode when Billie's sister Kristin was found, i would have pointed out she was evil the whole time. I would do this by being a guest star as a new witch just like Paige was.
HYE wanted to be alongside the sisters when they were kicking 'ass'