The most challenging campaign.

In my opinion, the Inferno campaign in the original Heroes of Might and Magic V might seem the most challenging at first, as a player accustomed to the easy Haven campaign doesn't immediately adapt to all the difficulties... But overall, I think the most difficult campaign is the one for the High Elves, due to the mission involving Nicholas... However, paradoxically, it's also my favorite... It features a large map and many undead creatures, providing an opportunity to test all the Elven abilities... I've completed it 10 times, trying all the difficulty levels... Yes, if you install the latest patch, with the dark energy for the Necropolis faction, it might seem less interesting...
I struggled a lot with the last mission of the Hammers of Fate on Hero difficulty... The Demon used to arrive at Duncan's location a day before me and kill him at the very beginning... I restarted it about 6 times... This is the only bug that really annoyed me...