Please Welcome to the Stage: Every Single Surprise Taylor Swift Guest

Who?! Maybe you know her from her Disney Channel past, or from her on-and-off relationship with Justin Bieber, or maybe from such hits as "Come and Get It" and "The Heart Wants What It Wants."

Where?! At Tay's final show in LA.

Why?! She was Taylor's costar in the "Bad Blood" video, but these two have a friendship that spans years, going all the way back to 2008 — that's why she's number one!