Which faction is the most interesting to play as, and what strategy do you find most engaging?

Interesting topic! One could discuss this topic for a very long time. Still, I can say that I have always been inclined toward Demons since the time of Heroes 3. In Heroes 5, what impressed me most was the opening of the Gates. With this skill, you can win a battle without significant losses, as well as block the enemy's ranged shooters. But in Heroes 3, Demons were still better.