Which creatures from the alternative upgrade options do you choose?

Haven
All red! All red!!!

Inferno
I rarely play as them, but I have a pretty clear idea of which units to upgrade:
Imps - +1 to speed, and they also give mana to the lords.
Greater Demons - for obvious reasons.
Hellhounds - they deliver the first powerful blow to the small units gathered in the corner.
Succubi - the damage from the chain lightning isn't that great...
Hellsteeds - versatile, unlike Nightmares, and generally stronger.
Cave Lords - not only because of their magic, but also because of their cleaving sword.
Archdevils - firstly, they summon lords, and secondly, Archdemons don't have time to reach anywhere (especially if the enemy has Storm Wind).

Necropolis
I often play as them and have a very clear idea of which units to choose:
Skeletal Archers - they are archers, after all.
Plague Zombies - the weakening attack is very helpful when sieging a city (they usually live there), and they also deal more damage.
Wraiths - again, because of their speed.
Vampire Lords - +5 hit points, +1 speed, stunning attack. This is only not helpful against undead.
Elder Liches - a guarantee of a lossless victory in battles of medium difficulty.
Death Knights - a more useful ability, stronger.
Astral Dragons - one bite, and the target moves every other turn, and even then, not very well.