wow, there are so many it's kinda hard to pick a best! love all crime docs and and movies too. Prison docs and mental institutions docs too, so I guess I'm a little crazy and have a criminal mind lol!

Try Inside Evil with Chris Cuomo. At least two seasons have aired so far, 5 episodes each, with a 3rd approved but delayed due to covid (last I checked).

In short, Cuomo interviews murderers behind bars, as well as some of the victims, law enforcement, and medical personnel involved in the subsequent cases. Pretty fascinating stuff, I stumbled across an episode on HLN and then ended up watching all the episides I could find. 

For those of you on Facebook there's actually some pretty good small series done on Facebook watch.  Kendall Rae, Explore With Us, True Crime Daily, Buzzfeed Unsolved (they got paranormal stuff too which if you like that, that can be funny), Reel Truth Documentaries, and if you like make up too... Murder and Makeup Monday (search that, not sure the girl's name think it's Bailey somthing that starts with an S.)  Quick little bites of crime drama that I enjoy over my lunch breaks at work!

It's not really a documentary, but The Night Of is one of the most accurate depictions of the NY criminal/legal system I have seen, moreso than even Oz. 

I really enjoyed making a murderer although too many documentaries end up being biased 

I recently finished watching a very interesting documentary about Troy Kell called "Gladiator Days: Anatomy of a Prison Murder".
A very graphic documentary about how Troy and another member of the Aryan Brotherhood plan and kill a black prisoner.

You guys watched "Don't f*** with cats" on Netflix? It's crazy, basically a group of animal rights folk on the internet hunt down and find out a guy who uploads sick videos killing animals, who as time passes makes threats on starting to killing people. Long story shot these net guys locate him, report him to the authorities and they do nothing. Next thing you know he kills and slices a guy.

Don't f*** with cats was great.   Just watched one on the Smiley Face Killers....pretty interesting although not sure how much I believe their theory.    Basically, it's about an unusual amount of college-aged males being found dead in a body of water after going out with friends.   The shows investigates 6 cases that likely weren't accidental deaths.   It convinced me that most of those cases weren't accidents but I'm still not sold it was by the same coordinated group of people roaming the country or having cells set up throughout the country.   A lot of the deaths were eerily similiar though who knows.

I really enjoyed Werner Herzog's documentaries on Life on Death Row. Gives you a good insight on Death Row, which I have always found fascinating. Into The Abyss is a good one. 

I'd recommend Popeye: Confessions of a Hitman.
The documentary focuses on John Jairo Velasquez aka Popeye, lieutenant to Pablo Escobar.
About him killing 257 people and arranging over 3000 more kills, his arrest and life after prison.

I see someone mentioned the documentary on Waco. The one on ruby Ridge is the same paints the government in all sorts of bad. How They Were Caught is a good documentary series on how  specific serial killers were caught. 

There was a Polish guy who have been a hitman for Roy DeMeo's crew, he had killed so many that he lost count and on top of that he had a normative family life and managed to keep his work as a hitman secret. The interesting part in all this was how he always looked to progress as if he was an intern dreaming about becoming a boss in lawyer's firm.

For those interested HardNuts is a doco on Australian gangsters and how they ran crime from the more professional criminals in Sydney to what was like the wild west in Melbourne.