Which installment of the Heroes series had the best campaign storyline for you?

Whether you agree or not, the “The Gathering Storm” campaign in Heroes of Might and Magic IV is the most original. You have to complete five campaigns, developing and leveling up the main hero in each. After each campaign, you get one hero. Then, the sixth campaign becomes available, which you have to complete together with these five heroes who met there. Each of them needs to have been leveled up in the corresponding campaign. In the sixth campaign, new super-monsters appear, like the Gargantua. I haven't seen this kind of campaign structure in any other version. Correct me if I'm wrong.