I voted for chapter 10, mostly because nobody else did, but also because I really liked the battle with (SPOILER)Cid.
Chapter 1: Where you start off.
Chapter 2: Vestige
Chapter 3: From sparkling crystal...
Chapter 4: ...to useless junk.
Chapter 5: Master & Padewan
Chapter 6: All this dampness is damp
Chapter 7: Flanitors
Chapter 8: Chocobos
Chapter 9: Collecting frequent flyer miles
Chapter 10: The Leftovers of the Other Chapters
Chapter 12: Back we go!
Chapter 13: The End
I voted for chapter 10, mostly because nobody else did, but also because I really liked the battle with (SPOILER)Cid.