Haven't seen anyone mention Marianne's paralogue yet. It wasn't horrifically difficult but I found myself rushing through it when I would have liked to spend more time farming materials from the beasts. Maybe my strategy wasn't up to par but the amount of beasts on that map quickly overwhelmed me.
Since FE16 is fresh in my mind, Miklan's map on maddening was also an absolute nightmare. Gilbert triggering the archers in the centre of the map made every single enemy on the map, with the exception of Miklan, start to move, so by the time you got to the top you had a horde of units to beat on top of the sword reinforcements with Pass. It was pretty much impossible to do unless Gilbert suicided at the beginning.
Also, I'm seeing a lot of people mention FE4 maps. Honestly, I went into FE4 expecting to hate the huge maps but it ended up being one of my favourite things about the game. The size of the maps really brought the narrative of every chapter to scale, in my opinion. Every event felt significant and impactful and having to traverse the terrain the way you did really made it feel like every chapter was integrated into the geography of Jugdral. I suppose it was more of a narrative thing for me.