My favorite faction in Disciples 2.

They are all optimists. With a sense of humor.

They certainly have a good sense of humor! I remember how the demons in the add-on committed genocide against the elves:
"Got you, pointy-ears. We will torture you slowly, cutting off one ear per hour until you tell us the location of your Avatar."
"We won't tell you anything!!!"
"We'll see how you talk when we feed you to the dogs, piece by piece..."

Although I can only judge based on the "Rebellion of the Elves" add-on.

I played "Rebellion" and "Guardians of Darkness." I don't know about other races, but I didn't notice any particular difference with the demons.
I like that in the Clans, almost all units can develop an area-of-effect attack. That's what I always use when playing as dwarves. What I don't like is their low initiative.
The elves in "Guardians of Darkness" are simply stunning, in my opinion. They are exactly as they should be: pompous, arrogant, self-absorbed, and warlike.
And I'm really enjoying the humans. Nobility remains only in the descriptions of the units. Finally, they are not the saviors of the world, but just people with all their virtues and flaws. And with their own quirks.

By the way, it's quite an interesting solution that there are no purely positive, white, and fluffy races in "Guardians of Darkness." They are all good. The elves, the horned ones, the undead, and the dwarves are all trying to commit genocide against each other at the first opportunity. And the humans, against the backdrop of this chaos, are trying to restore order in their state by killing each other. And the active participation of the gods in all this adds to the charm. The Almighty is devouring popcorn by the bucket, Betrezen is raging, Wotan is creating something incomprehensible while drunk, Ghallean urgently needs to see a psychiatrist, and Mortis needs a cute vampire.