I think the first episode 'The Mad Woman in the Attic' is the best of the series. This is the only episode where we can guess if the prime suspect is really the killer or not. He suffers amnesia and says that he cannot remember, but is he telling the truth or is he pretending? The writing and acting are brilliant, which lead the audience into doubt like the police in the film also experience. It reminds me to the first episode of Prime Suspect (Did George Marlow do it or not?) and first episode of Touching Evil (They don't have enough proof to charge Professor Hinks).
In the rest of the episodes (2 - 11), we can see the culprit(s) even before the crime is executed. So it's more about what and why. I myself like detective shows which, not only explains what and why, we can guess who is the killer.
Christopher Eccleston's character dies in episode 4 and what a pity because he is one of my favourite actors. Here in Cracker, he was still young and not as thin as he is now.