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1) One of the main indicators in the game is the speed of development. How quickly can you capture resource mines, how quickly can you build an army that is so superior to the enemy that you can storm 2 Castles...?
Here, I would rank them in this order: Necropolis, Humans, Serpentfolk, Orcs, Demons. With the first three, by combining the armies of 2 heroes on the 1st turn, you can confidently go for the mines even on a high difficulty level.
2) Mobility - who outmaneuvered/outpaced/didn't catch up with whom. Humans are first, Undead are last (leadership is key).
3) Army flexibility. Undead are in first place. Out of 7 units, 3 are ranged and 3 are flying (storming Castles is a pleasure). They also have 4 units that deal physical damage and 2 that deal magical damage, 1 of each to choose from - you can focus on whatever you want...
As a result, Undead are the most attractive, but since they move very little (compared to others), you have to give them up.
Humans deal good damage with light magic, which works against 3/5 factions. They lose to Serpentfolk...
Serpentfolk lose to Necropolis if the latter have a good strategy and don't let the ranged units get close.
Demons are essentially a melee race; they are good in open fields, but not very good at taking cities.
Orcs are a powerful race, but the lack of healers (like Demons) slows down development in the early stages. If you let them develop, they will crush everyone, but if 3 weeks in, a mega-pack is already standing at the walls...
I vote for Undead. Maybe I'll find a solution to the logistics problems; otherwise, they are the best. IMHO, of course.