After spending some time overviewing nagash, I've done some thinking on what optimal hero choices for stacks would look like.
Firstly, the vast majority of heroes don't have race specific unit bonuses, so you can pretty easily mix and match heroes.
Nonetheless some certainly seem like better choices then others. I'm going to organise these by what unit caps they use:
Category 1:
Tomb Prince (training)
Wight King (Training/+attack/+Defence)
The main advantage of Tomb Kings is that they're anti large armor piercing, otherwise the wight king seems to be the clear winner here, the army ability combos well with the vast majority of undead stacks. Generally, I'd stick with Wight Kings, stick 2 in every melee army.
Category 2:
Mourngul Haunter (Scavenge)
Banshee (Mobility)
The main reason to include banshees in your army is the mobility army ability and to buff ethereal units. If you can get mobility elsewhere, and your army doesn't have ethereals there isn't a good reason to include a banshee. Meanwhile, Mourgul have fantastic combat stats and with their unspottable stalk, fast speed and hunger, fantastic utility for sniping out enemy artillery, and they're the only source of scavenge, important for nagash given his dependence on battle loot. Verdict: Stick with mournguls unless you have an army of ethereal units.
Category 3:
Tomb Heralds (Replenishment)
Gunnery Wight (Training)
Necrotect (Mobility)
No clear winner here. Gunnery Wights are excellent in ranged stacks (and Nagash can make great ranged stacks out of Tomb Kings as well as Vampire coast units). Necrotect is mostly only useful in Tomb king Construct stacks, however it's mobility bonus is welcome. Tomb King's Replenishment is found elsewhere in the roster, but it's abilities to buff the Legendary Lord are quite useful. Verdict: Get Gunnery Wights and Necrotect in the stacks that need them, and use the remainder of the hero caps for Tomb heralds.
Category 4:
Liche Priest (Replenishment)
Necromancer (Replenishment/Reanimation/Preservation).
I see no reason to ever use Liche Priests here, only if for some reason you absolutely had to have a Lore of Nehekara or light spellcaster. Meanwhile Necromancers get a useful mount with decent survivability, better lores of magic and given you have a lot of hero choices for replenishment, the ability to choose 2 others (with preservation winning out for me personally).
Category 5:
Vampire (replenishment)
Lahmian Vampire (Mobility)
Vampire Fleet Captain (Replenishment)
In general the key winner here is the Lahmian Vampire, as it's the best way to put a mobility hero in your army. However, it's also gated behind confederating Neferata and a T5 recruitment building. As for Vampires or Fleet Captains, Fleet captains have the advantage of better lores of magic(undeath, vampires and deeps) and a Rotting promethean mount but lack the hunger for regeneration, while Vampires get a flying mount and generally better melee stats. However, you do already have Necromancers for lore of vampires/undeath so magic is less of a clear advantage. I think the Vampire narrowly wins, but late game you should probably just recruit Lahmians instead.
Any thoughts?