Necropolis and Demon.

It all depends on what kind of game it is:
1) if it's a duel, then definitely Demon (because they have more advantages)
2) but if it's on a map, then of course, Necromancer)))
P.S. this is just my opinion...)))

In duel mode, Necromancers easily defeat Demons.
Deydra will most likely kill all three duel heroes: Nimues, Marbas, and Deleb.
Ravenna easily handles Deleb, but it will be a little more difficult against Nimues and Marbas, but I still give her more than 60% chance of winning!
Orson - here it all depends on the situation and luck...
He fights on equal terms with Nimues and Marbas, and it's a little easier for him against Deleb.
By the way, Orson is considered the strongest hero among Necromancers in terms of defeating all duel heroes, but in this case, he turned out to be closer to 3rd place.

On the map, Necromancers also have a clear advantage, although, of course, it directly depends on the skill of both players and the situation.
Even in a long game, the Necromancer's "meat" is not compensated by the Demon's summoning :D, because the Necromancer simply develops faster and better, and the Demons will also suffer losses.
Summoning will take away the initial initiative advantage - and then the vampires will strike, and the spirits with dragons will arrive + the hero will cast either Berserk or Subdue or simply weaken the Demon's troops + raising the dead is a very effective spell!