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In the fifth installment, Dwarves are clearly not the strongest. I don't see much power in them. Demons were the most appealing to me. Before Succubi/Demonesses, they were still in last place, but after, they became leaders due to the power of their 4-7 level creatures.
In the sixth installment, Inferno is one of the strongest factions. The only downside is the stupid and weak dogs, but otherwise, all the units are incredibly strong and well-balanced for team battles.
Personally, I really liked the Dwarves for their strength due to their rune magic, and the Demons are great because of their initiative and the quick summoning of Urgash through the gate.
But I can't even imagine how you're going to win with Demons, Necropolis, and Haven in the sixth game.