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In the second game, I rarely played through the Campaigns. In the third game, I did play through them, but the level of quality was different. For example, "Armageddon's Blade," in my opinion, is incredibly boring; all the missions boil down to "take a large number of your cities and use them to capture a bunch of enemy cities." I completed many missions, but I couldn't endure this nightmare. However, "The Rise of the Barbarian" in SoD was very enjoyable; I wrote the reasons somewhere in the third game section.
In the fourth game, I only played the expansion pack Campaigns (of the first and second expansions), and I didn't notice anything interesting.
In the fifth game, there's a high-quality and well-thought-out plot; they often give interesting tasks, and the cutscenes create a unique atmosphere. The endings are a drawback; they don't seem to know how to make them. The fact that all the heroes become kings and queens is comical. But, setting that aside, it has everything: clever plans and intrigues, psychology... I voted for Heroes 5.
I'm hesitant to draw conclusions about the sixth game, but what I played (the prologue, the first mission for Irina, and the first mission for the Orcs) didn't leave a pleasant impression.
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As for the "Chronicles," there were some good Campaigns, as I recall. Although I only completed one to the end: "Master of Elements."