As far as death stars go dark eldar and eldar have the most options for death stars out there. With several unit options and several character interactions that lead to some of the best ones in the game.
What a good death star requires in varying levels is: durability, versatility, mobility, and board presence.
The most used character in regards to
de/e death stars is the Baron. With stealth, the phantasm grenade launcher and hit and run he is a bag of tricks for a death star. Stealth helps with durability, grenades help because many of the dark eldar and eldar units lack grenades, and hit and run helps with mobility, as well as the main problem many people have with death stars which is getting tarpitted by weak throw away units.
The other characters that are usually vital for these elfin death stars are farseer support. THe powers available can really push the envelope in terms of hitting power and durability. Prescience letting you reroll all hits is huge, fortune letting you reroll all saves(2+ rerollable shadowfield is nasty), and invisibility can be game breaking against many armies rendering death stars immune to most general shooting(tau and eldar excluded). The shard of anaris on the seer is a great development with the new eldar codex letting you make the seer fearless. Fearless is a big deal for a good death star.
Fearless super killy combat characters. Eldar and dark eldar have access to quite a few of the nastiest characters in the game. Many of the phoenix lords are highly usable in this regard, not only are they killy but often bring some extra rules along with the aforementioned important fearless rule. But out of all of the characters the two that stand head and shoulders above the pack are Asdrubael Vect and Karandras. Vect is a total tool box with fearless, a haywire grenade, and 6 base attacks at i8 wounding anything in the game on a 3+ at ap3, the only things he doesnt kill are 2+ armor saves, and he has prefered enemy everything which passes onto his unit... nasty. Karandras is perhaps even stronger, with an i7 powerfist he has 5 base attacks at
str 8 ap2, stealth for his unit, and monster hunter for his unit as well.
As far as units go there are several that to varying degrees operate as effective death stars.
At the bottom end is a group of 10 wraithguard or wraith blades. They are tough, have good
str and versatility with either guns or the close combat options. Probably need baron, a farseer, and a killy character for full power options. Can also benefit from a spirit seer or several(INyaden) to both make them scoring(scoring death stars add versatility) and perhaps get them that 2+ armor save. Downside overall is speed, and low wound count.
Grotesques are next. Also tough with t5, feel no pain base, bunch of wounds and good
str and number of attacks. Need baron and some other support, as there are some targets that they cant really hurt(dreadnoughts etc.) They are also slow, and have no real armor save.
The harlequin star was something run in early 6th when fortune was a guaranteed power. Might not be as worth it now. Its benefit is it already has hit and run base so more room for strictly super killing characters and not needing baron.
The two biggest stars though are the jetseer council. 2 seers, baron, lots of warlocks. Its fast, can target lots of things, is hard to pin down and with fortune and lots of other powers its hard to tangle with. Its been winning a lot of events on the east coast right now. Run by a player over here its very well respected.
Now I didnt leave that for last because I dont use it
lol. I use a beast star. Ive used many variations of the star. From base in small games, to baron only, to baron and vect, to vect only. One seer, two seers, either is good. Fortune is great, invisibility might be better, both is super duper great. Cant use only kymerra it hurts the versatility and denies you the joy of razorwing flocks. Pre tau I was a 5 kymerra 8 razorwing guy, with tau and eldar now you need more kymerra to survive and cross the board, I use 10-15 and 6-4 razorwings now a days. Fearless is key with this unit, I use Vect because it amuses me to do so and he is extremely good for how I like to play. But a shard seer and baron could work as well for other people.
Overall the death star options add a bunch of other builds to the army. Eldar and dark eldar are tough to separate in my mind now.
AS a whole they have a wealth of effective options when utilized together. With 3-4 effective death star builds that are all different, full mech mainly dark eldar with minimal support, full mech eldar with no or minimal
de support, mixed mech either way, and full foot either way. Its a great time to be an eldar dark eldar player. At the battle for salvation tournament
http://www.dakkadakka.com/dakkaforum/posts/list/550182.page the top 3 armies by battle points were all eldar base armies. 2 of them were allied to dark eldar, and all were highly different lists.
Winner:
Farseer, Jetbike, Runes of Warding, Runes of Witnessing, Spear (Doom, Death Mission, Fortune)
Farseer, Jetbike, Runes of Warding, Runes of Witnessing, Shard of Anaris (Prescience, Precognition, Misfortune)
10 Warlocks on Bikes, 4 Spears
5 Squads of 3 Windrider Jetbikes
2 Squads of 6 Swooping Hawks
2 Wraithknights
Skyshield Landing Pad
Baron
5 Kabalite Warriors
Second Place
Farseer with Spear (Shriek, Terrify, Invisibility)
5 squads of Dire Avengers in Wave Serpents with Shuriken Cannon, Scatter Laser, and Ghostwalk Matrix
3 Squads of 6 Swooping Hawks
Rune Priest with Melta bombs and Runic Axe (Jaws, Lightning)
Lone Wolf w/Terminator Armor, Chainfist, Storm Shield
8 Grey Hunters with one Melta gun
Land Raider Redeemer w/Multi-Melta
Third best battle points
Farseer bike, spirit stone of anathalin
Farseer
3 Jetbikes
3 Jetbikes
20 Guardians - 2 bightlances
Wraithknight
Wraithknight
Vect
20 Warriors - 2 splinter cannons, sybarite(
PGL)
20 Warriors - 2 splinter cannons, sybarite
5 Beast Masters - 10 kymerra, 6 razorwing flocks