Gameplaywise Dawn Brigade because their chapters in hard more are the perfect difficulty for me in FE.
You have to care about some fragile units including the lord, Micaiah against enemies with serious stats. I love 1-3, 1-F, 3-6 and 3-13 because of their difficulty.
Only 1-9 is annoying if Micaiah is bad.
CK's and GM's have some units who are much higher level than the enemies so their chapters are less challenging.
However part 2 features two of my absolute favorite chapters in FE: 2-P and 2-E