I think the very first skeletons were better.
Finally, I found a useful article about necromancy and dark energy points on the English wiki. It turns out I was wrong - the number of skeletons does increase, but it's not multiplied by the level difference; it's multiplied by the ratio of HEALTH, so a bulky Assassin for the first level was resurrected not by 20%, but by 35% of the number, and fewer Death Knights were resurrected from weak witches.
In principle, you can reduce the amount of dark energy at the start and from the necromancer's pillar, and increase the points gained from the level.
