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

I think you should play as either the Demons, focusing on opening gates, or the Orcs, as they are strong with a good army and the "Wrath of the Horde" ability. The Elves have good racial abilities. For the Undead, you can quickly build up your army. For the Dwarves, you need to be good at using runes. The Mages have many archers and strong, large creatures. And the Dark Elves, if you take Assassins and Shadow Lords (level 2 upgrade) and upgrade all the others to level 3, will have a strong army. In general, when building an army, you should choose the highest possible upgrade level.