The most challenging campaign.

Personally, I found two missions challenging – the 3rd mission for Inferno and the Emerald mission for the Elves. But that was when I played for the first time, knowing practically nothing about the 5 hero types, the skill tree, or the tactics for playing each race (although I had played other Heroes games, starting with the 2nd one). Later, when I replayed the game on a higher difficulty level, I didn't have any problems anywhere, as I knew the correct strategy for developing and upgrading the heroes.