The most challenging campaign.

for all the most “challenging campaigns,” there are small nuances that make them not so difficult – this is the development of your hero, which may not always be suitable for all campaigns, and the tactics used.
For me, the most difficult one was (perhaps of all the times I played through all the Heroes campaigns) the battle against Godric (in the castle) on the second month – he had a mana absorption amulet + antimagic + divine retribution + guardian angel + leadership... that was quite a challenge – one wrong move and 40 minutes of real time would be wasted.