The Endless are pretty much THE top of the food chain where conceptual entities are concerned -- and Death is basically the most powerful member of the Endless...
Death of the Endless is the final concept/being that will be left when all of creation finally comes to an end. She ultimately outlasts, and 'kills', even Destiny himself(along with with the whole of creation and ALL other beings/concepts, of course) with little more than a gesture:
http://i.imgur.com/cLZvfdX.jpg
http://i.imgur.com/jbLpmHI.jpg
http://i.imgur.com/niZI8GS.jpg
So despite the fact that there are multiple personifications of Death in the DCU, Death of the Endless is undoubtedly the most powerful/important among them, given that she --and she alone-- is the last thing left when ALL creation ends.
As for where 'lesser' Death abstracts, like Nekron, stand in relation to Death of the Endless, I have always found this scene quite telling...
Lex: "The attack of the Black Lanterns! The dead rising! That's what this is about, isn't it? It was you behind that!"
Death: "Erm... Oh. Yeah. I noticed that. It looked like they were all having a great time. You know, people do come back from the dead. It's not a big deal... I AM kind of busy.":
http://i.imgur.com/QO5IJRB.jpg
Lex: "The dead have come back to life! Several of them!"
Death: "Like I said: BUSY It happens! In the end, they ALL come back to ME.":
http://i.imgur.com/6xhLI3n.jpg
**Death certainly acted like Nekron and the entire 'Blackest Night' event were utterly insignificant and barely warranted her attention.
Imo, this is noteworthy because Nekron alone is a bonafide universal death-abstract... On top of that, he is very similar to Marvel's Oblivion in the sense that he also embodies absolute nothingness...
Nekron IS the sentient darkness/void that existed before the DCU was even created:
http://i.imgur.com/QrnQBNq.jpg
http://i.imgur.com/uWKunpv.jpg
Nekron IS "the empty space between atoms and planets":
http://i.imgur.com/z46gF16.jpg
Heck, even Black Racer(unleashed) is a universal death-abstract:
http://i.imgur.com/gZdJvt4.jpg
http://i.imgur.com/JOQsoGi.jpg
Also remember that Black Racer and Black Flash are the same being:
http://i.imgur.com/FdFeuMt.jpg
http://i.imgur.com/KZAZmsp.jpg
...And by all indications, Death of the Endless is above them ALL.
Just some food for thought. kruemelmonsteryn0