Best: FE8 Chapter 19 (Hardmode, of course) I absolutely LOVED that giant clusterfuck of soldiers near the throne room, I always have fun with it.
Honorable Mention: Battle Before Dawn (Hardmode, again) For similar reasons to FE8 C19. Though I'm surprised nobody thought of using a warp staff for the chapter, it worked well for me...
Worst: Arcadia
Why are all my powerful units suddenly useless? Oh yeah, it's a desert. Hey, WHERE'D THOSE WYVERN KNIGHTS COME FROM? WAIT, I HAVE TO PROTECT SOPHIA?!?!?! YOU ARE TEARING ME APART FIRE EMBLEM!
Honorable Mention: One Survives
Needless to say running around with the Black Knight was cool the first time around, but this map was just TOO BORING.