I'd say Chaos Dwarf warriors as an overall choice. Without them it wouldn't really feel like a Chaos Dwarf army, and they're essentially blocks you can use to slow the enemy and hold them for several turns if things go well.
In a themed Hobgoblin army however, led by a Hobgoblin hero and comprised of armoured wolf riders and armoured Hobgoblins, I'd reckon Hobgoblins would be the no brainer choice for the role of 'essential unit'.
my general himself and his squad of 19 warriors.
I've played 4 games and we was purly awsome in 3 of them (the last game the opponent spent the entire game running from my generals unit so they never saw combat
).
first game he took a flank charged by a large unit of night goblins with nets. survived 4 rounds of combat never winning till help arrived (damn nets, making my dwarfs strength 2)
second game was charged by a large unit of orcs, dueled the enemy general and cut him down, than procedded to slaughter the orcs
3rd game charged a unit of dwarf warriors and challenged their general and cut him down
Flying terror causing monsters for chaos dwarf heroes are great.
Blunderbusses. Given the 3 compulsory core CD units at 2k points, I think it's fluffy to give 2 out of 3 of my "fire dwarf" units firepower. Other than that, 2nd place would have to go to the Bolt-throwers, I have played without most other units at times
I'd say hobgoblin wolf riders (for harassing the enemy and drawing them within range of my blunderbusses.)
with blunderbusses in a close second.
Sneaky gits. Because they're there
I use dwarf rules for my CD since they aren't tourney legal anymore according to our local judge... so I'd have to say either my runesmith/sorceror lord, or my cannon. I get beasted by chariots a lot and the addition of that cannon has really helped keep them away from my lines.
I would have to say at least one unit of cd's with great weapons. i just can't play without them