The true crime case that always got me was Mr Cruel, a serial rapist, home invader and later child murderer who operated in Melbourne during the 1980s and 1990s. There are four "canonical" attacks on children ascribed to him from 1987-1991 (after which he went cold), but he is a suspect in the rapes and abductions of up to a dozen women and girls in Melbourne at that time, and is probably one of the most demonic sexual offenders I have ever had the misfortune of hearing about, second only to Junko Furuta's murderers.
Even though he's still unidentified he had a specific MO that helps narrow down which attacks he committed; he would monitor his victims for some time, enter their home and subdue anyone else inside, abduct the victim (in his earlier attacks he raped the girl inside her home before leaving) and bring her to his home before releasing her after a specified time he would tell his victims. He always wore a balaclava (the sketches of which are nightmare fuel), stole miscellaneous personal property from the home (usually clothes), cut power lines to and from the home and would generally misdirect the victims in some way, such as pretending to talk to someone else in their presence. His canonical victims were all between 10-13 years old. One of them, ten year old Sharon Wills, saw that he was filming her rape during a brief moment when he was away from her (he blindfolded all the victims he abducted). He was extremely paranoid about being identified and would wash his victims in a tub and force them to brush their teeth after attacking them. He had particular "catchphrases" as well, the most repeated one being "My liberty is more important than your life".
His last victim, 13 year old Karmein Chan, was the only one of his victims to be murdered by him. We still don't know why he killed her in particular, and afterwards he never struck again as far as the police knew. She was abducted in 1991, and her remains were found a year later in a ditch. She had been shot execution style.
It's possible Mr Cruel is still alive as he was estimated to be anywhere from his late teens to mid thirties during the attacks - he was described as having a slight pot belly and body hair but his voice was quite young. In the event that he's still alive the police believe he's an avid pornography user (especially of CP), may be engaged in trading it and may even interact with children over the Internet. There was some misinformation that he may be a school teacher as the canonical attacks occurred during school holidays but this was debunked by Melbourne Marvels.
If you guys want to know more about this case I would recommend the Melbourne Marvels podcast and website and the "Who is Mr Cruel" wordpress blog. This website also has a lot of good material on Karmein Chan's abduction in particular:
https://thecruelsong.com/2024/08/01/who-was-karmein-chan/#canon-attacks
I do hope that this case is solved one day; the arrest of a suspect in the Australian Easey Street murders in the 1970s and that there might be advances in the Madeleine McCann case stop me from becoming doompilled every time I think of this case.