Zankyou no Terror, Hero Mask Recommendations
Both shows are detective series, where detectives tracking down a group of criminals, who were part of a secret experiment. Both have similar dark and thriller atmosphere.
Both series deal with people (prisoners in the case of Hero Mask, orphans in the case of ZnT) undergoing special treatment/training to gain almost super human habilities. They both have a very similar approach to the subject matter, combining mind games and investigations with some truly thrilling action scenes. All in all, they share a similar feel and atmosphere, so I'm sure you'll like one if you liked the other.