This probably belongs in the general discussion section, but the mods can handle that.
I only have the DE codexes back to the 5th edition one so I can't comment on how the current covered compares to 3rd/4the, but I absolutely love our new codex. Lot less wargear options than older codexes like 5th, but the army is probably the strongest it's been in a very long time, and the ability to get different abilities between the kabals/cults/covens is awesome. The best thing is honestly these days almost all our units are viable. Yeah if you're a tournament player some builds are definitely more competitive, but for standard play you can make not of our units work.
If you want to run a pure kabal, cult or coven kabal is definitely the strongest due to all of it's firepower, but any combination can work. Personally I think a strong kabal detachment backed up by a sizeable cult and/or coven melee element is the most fun and interesting army, but you can do whatever you want really.
The Night is Dark and Full of Terrors...