Which skills are best for a hero?

In a long game, it's worth taking "Soldier's Luck" – if only because it can't be replaced by any artifacts or buildings.
Regarding the Princes – "Soldier's Luck" affects the probability of the Princes' "Stun" ability triggering. This has been proven for a long time.

Because of just one "Soldier's Luck," it's not rational to occupy an entire luck slot with it. Especially in a long game, when each skill slot is literally worth its weight in gold when upgrading skills. This is only if you really have no artifacts, no utopias, or corresponding buildings – then yes. And what does "Soldier's Luck" give? It only affects the triggering of two abilities in a Necromancer's army – stunning skeletons, and if we believe what Antonag07 said – the percentage of blinding vampires. And even in the latter case, "Soldier's Luck" may have an effect, but it's barely noticeable. At least, vampire princes blind quite well even without it.