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

Interesting topic! You can discuss this for a very long time. But still, I can say that I have always been drawn to demons, even back in Heroes 3. In Heroes 5, the most impressive thing was the Gate ability. With this skill, you can win a battle with minimal losses, and also block enemy ranged units. But in Heroes 3, demons were still better.