Hmmmm... Wait... how was Florina in the top 3 worst? O.o
Best: I don't really like defense chapters so I'll just say FE9 chapter 8 (the one with Ilyana) it's challenging and a good source of exp. for units.
Worst: I agree with others, chapter 15 Hector mode was rather lame. It's not only the boss it's also the enemies in general, chests that are rather far IIRC and stuff.