The most difficult and the easiest scenario.

For me, the most challenging were:
Battle Cry of Freedom - a terrible mission with tons of enemies, it was very difficult for me.
Dark Messiah - The beginning is very difficult, especially for a hero, and sometimes you can't even survive the first attack on the Necropolis, I struggled with it for about two days!
Diplomat - I cursed this mission because of the annoying condition of not fighting with mages; they even invaded my lands and almost attacked my city, but I managed to complete the task in time, although it was a close call.
The easiest ones were:
The Last Flight of the Falcon - The first scenario I completed and my first game as Demons =) (I even remember running around with Jezebeth when she still had the skin of Biara).
Trial of Agrath - It wasn't exactly easy, but it was of medium difficulty; the main thing is not to make mistakes and clear the map.
Alliance - Also a very simple scenario.
Maahir's Gambit - One of the easiest scenarios, in my opinion; the Necropolis doesn't really have a chance, especially in Hammers of Fate. Zoltan with 2,000 skeletons just fell apart.
I can't even remember the others; I didn't have time to play Hammers of Fate properly.