Why only Necropolis? The topic is in the "NECROPOLIS" section, but I still think that Necropolis fans always enjoy reading about death, and especially about the death of units in OTHER castles =)

So, in order:

Haven:

Archangels - no comment =) Beautiful and memorable.

Inferno:

Archdevils - it's not bad, but it can't be compared to archangels, of course...

Sylvan:

Ents =) Just because there is no other choice)) In principle, ents die quite nicely.

Necropolis:

Harbingers and Vampires. They have already been discussed here, and I have said everything I wanted to say =). I would also mention Liches - they are not bad either, although they die quickly...

Dark Elves:

Cave Hydra dies beautifully). However, the lizard makes incredibly horse-like sounds when it dies, which cannot fail to attract attention 8)

Finally, Academy =)

I think that the most work was done on the death of units from the Academy. Gremlins, gargoyles, and golems die quite normally, but starting with mages, the situation changes. Mages die for quite a long time, constantly trying to clean their robes from dust/dirt/blood, etc. Djinns, in my opinion, die very beautifully, I would even say that they spin back into the lamp, just like in the tale of Aladdin =). Rakshasas lose the shine of their limbs when they die, and as for Titans... In general, the best race in terms of unit death animations. Period!