Is Necromancer the most powerful and versatile unit in King's Bounty series?

[Autoclave]

I always see this "no loss games". Do you mean not loosing individual troops during battles?

All ghosts have soul draining ability which makes them replenish their troops after doing damages. They can even go above their current numbers. Vampires have something similar in bat form, but they cannot go above the original stack size.

So with careful playing you don't have to worry about ressurecting these two.
Put target on ghosts, stoneskin, and you have an almost unkillable unit by physical damage.

As for the spells: You have eviln and turn back time. Eviln is probably the most incovenient ressurection spell, but it is pretty powerful. There is also necro call that raises your dead troops. I don't know however if this counts as a ressurection or a summoning that vanishes after the battle.