Well, having read more attentively the DE scroll, here are my findings.
1. Monsters.They are deadly. 9 to 15 attacks, hitting average 4+, wounding easily 3+, often making 2 damage.
They have 10-14 wounds, K-beast, Hydra and Maly can regenerate them.
In addition, dragons fly at 14".
If you want to give your opponent hard times, go monsters.
At first glance, I'd rate:
Malekith > Dragonlord > Manticore BM > Hydra > Kharibdyss > Dragonsorc.
Note that the MantiBM makes reroll hits of 1 by monster models (including the rider). It is only because of this ability that I rate him better than the non-flying monsters - but only to make another monster better.
Note that for some obscure reason, the Manticore can't fly.
2. ShootersRBT at 36", shoots 12 times. I understand it counts as 3 models. Can't beat that.
The scourgerunner seems interesting, it moves 12" and shoots 5 shots at 16 or 18". Plus slight bonus against monsters.
Note: Strange enough, it is now a unit of 1-99 models, including one kind of champion named BM. I see no benefit to take several of them, I see only benefits to say that all SR are single unit with BM.
However, COC are quite good as shooters and twice as deadly in melee. They are better choice if you have not built your model.
DR are good with 3 shots and 14" move. Plus interesting ability. Take shields, always.
Shades could be interesting as scouts, but I don't foresee any great use for that ability. Still, they shoot better than Darkshards.
EDIT:
Lord Drakon AOS reports has a good use: to carry assassins close to the opponent's heroes.
Darkshards have one great ability: the minimum size is... 1. And that 1 can be the champion. Using this flaw makes them immune to Battleshock. I'm sure there might be other loopholes in melee and shooting phase thanks to this.
RAW for Medusa/BWS indicate that if a unit is within 10", all models (not limited to those in range) have 1/6 chance to get a wound.
That seems a great anti-horde ability: a unit of 4 medusas halves the number of models. However, it makes no sense to kill models out of range and I would not personally use this ability for models at more than 10".
Overall, for shooters, I'd take RBT and SR (if model already available) and DR above footers.
3. Mass melee.Here, I look for large units, with the ability to add the attacks from many models.
And the winner is... the COC, by far. One unit can include 99 chariots if you have them! And with 6 attacks per model, it hurts!
In addition, they are not bad shooters and they move fast. Their only drawback is their width, which prevents rear ranks to fight. You could field a large unit of COC in a very wide line, and charge! It would stampede most things on its path.
Be sure to take a CO dreadlord as general if you go for such a large COC unit.
COK are fast and deadly (especially on the charge). They have a standard. Good save. What not to love?
Warlocks seem to be good in melee, very quick, with 4 attacks per model. Plus arcane ability! They should be great to protect your flanks/rear.
The Medusa, in unit of 1-99, could be a neat choice, with many deadly attacks per model, in addition to its deadly shooting. It is quick, too. Its main drawback is that is is not an Exile/DE unit, so no command ability can help its mediocre bravery.
Then come the footloggers. Their advantage is that they can be compacted in large, dense units.
I'd evaluate roughly that WE>BG=Execs>SoS.
WE supported by a CoB have an incredible density of attacks. However, they will die in droves with no armor but the CoB.
BG and Execs are close. BG get the advantage of a greater weapon range.
SoS are not as good as WE with support, but at least they need no support.
The corsairs need obviously to be commanded by a Fleetmaster of Lokhir. Even then, I'm not sure they reach SoS level.
I see no use for dreadspears/bleakswords, unless you take the Exiled Warhost - but in this case, you could take only 3 models of darkshards and get away with them...
EDIT: Spears could be a nice defensive unit, in very large numbers.
4. Single unit melee.Note that heroes are limited to 1 per unit. It means that it is not interesting to buff them as units.
However, no rule prevents you to take 99 units of dreadlords if you wish.
And that would be very useful.
Dreadlord on foot or on CO (including Malus) are very capable individual fighters, much deadlier than any WE or GB.
If you're not limited by mutual agreement or comp, do not hesitate to take as many as you can.
The assassin (not Shadowblade) has interesting anti-hero ability. It can hide in any DE unit, any number of assassins. You could imagine "air Ninja" dropping from a dragon a dozen 'sass in vicinity of an opponent hero (possibly riding a monster) and all the sass poisoning that hero before it has a chance to strike.
I'd rank the single units as follows:
All monsters > Malus > CO lord > foot lord > COB Hellebron > BW shrine. You can take them for their fighting ability.
Then, Shadowblade > Assassin > Hellebron > CoB = COC > Lokhir > Master BSB > Fleetmaster. If you wish to take them, they fight better than any ranked unit, and in addition they bring unique properties.
All other single model units are to be taken because of their unique ability, not because they fight well.
5. Buffers.Noted already the need to take a CO dreadlord to help COC/COK to be even deadlier.
Hellebron allows one unit to pile and attack twice. That is incredibly powerful in conjonction with a large unit.
The dreadlord on foot has the great ability to have one unit reroll failed wounds. This alone makes a mediocre unit become elite.
The BSB has a similar ability, and this is not a command ability! One should always take a BSB.
Note that RAW do not prevent anyone to take several BSB. That seems strange, when only one general can use a command ability. I'd assume that BSB should be limited to 0-1, even if it is not specified.
I have not realized yet the usefulness of other buffs or magic abilities.
Please complement that first study.
6. Magic-users.Well, they no longer compete against each other for scarce powerdice.
Malekith heals himself.
The sorc on foot has WoP wich makes an opposing unit hit less (shooting and melee). That can be precious.
All other casters are just like another shooting unit. With a good movement value, that can be useful sometimes.
For spellcasters, the synergy seems to be with terrains.
Balewind Vortex, Dreadfire Portal, Dreadstone Blight, Witchfate Tor make interesting additions.
7. Units with no use.Well, Bleakswords seem not to bring much to the army. I see no particular use for them.
Everything else could argue some use.