Darker than Black: Kuro no Keiyakusha, Zankyou no Terror Recommendations
The highlight of both show's main similarities are centered around the main protagonists:
They are willing to do wrong to achieve things which are right to them.
They have a dark past which constantly taunt them.
They are emotionless when it comes to violence.
They use aliases but their true names are unknown.
They are terrorists in the eyes of society.
Other than that, both shows somehow touch on society's problems but also involve conspiracy plots. They are also dark. Of course, the visuals are nice in both of these shows.
When it comes to differences, however, DtB is more fast-paced in action and has more characters to fill in the plot
Darker Than Black has the same sort of antihero motifs, highlighting how characters can be sympathetic and also have atypical motives for crime and resolution. The characters in Darker Than Black are more fleshed out, and the government interplay is much more prolific than in ZNT. Both shows have superb detective agencies working to solve the crimes, and they even share similar visual designs and how the characters work.
If you liked ZNT, you'll like Darker Than Black.