Teen Detective is by Richard Williams (who I do the Across the Table podcast with) and Best of Fiends which is a work in progress from Stuart Chaplin (and which is currently unavailable generally as far as I know, I asked Stuart whether I could take his notes, which is how I got a copy).
Teen Detective builds off Cthulhu Dark but I think I'm going to have to read my latest copy of the rules again because it doesn't feel like it has that much in common with it anymore. The closest intersection is around destroying evidence in an investigation.
Instead Teen Detective uses a system of gaining Edges over people by investigating the mystery. You also have a pool of points that allow you to get through moments of failure of imagination or inspiration.
You can also take a risk to succeed which switches to a simple d6 check but interestingly what you are staking is not your personal health or well-being but your family's dark secret.
The dark secret is created for you by the other players but the GM gets to choose the actual dark secret from two choices the player selects. It's a modified Archipelago mechanic and it is pretty smart and really plays well into the Veronica Mars theme.
By comparison Best of Fiends is a PbtA but thankfully avoids playbooks by offering a shared set of moves and motivations that are unique around the table. Stuart also nails the attributes of the genre with: Sweet, Dark, Trouble and Strange.
Another simple innovation is that the rules introduce Advantage and Disadvantage to the PbtA system.
Roll three dice and take the highest score if you're at an advantage, the lowest if you are at a disadvantage.
A hundred moves and special carries can now be excised from PbtA rulebooks everywhere.
Playing mysteriesTeen Detective spends a lot of time trying to accomodate different playing styles from solving a fixed conventional mystery scenario that a GM conceives, to a pulpy action game to the story-telling mode that feels like it's default gear.
It also is less interested in whodunnit but why and what the Teen Detectives are going to do about it now. Consequences are obviously interesting but the thing that struck me is that it doesn't seem quite on genre. Noir detectives quite enough try and create their own sense of justice but teen detectives are never really vigilantes and the shows never really go as far as saying that the adult world is irredeemable corrupt. Really it is the consequences for the characters that are significant, the guilty always seem to punished by the authorities in these kinds of stories.
Best of Fiends is entirely simpler: something bad happens, your character (and the others) all have different connections to it and the characters complicated relationships do the rest. It's not really about the mystery but like Monsterhearts is about what the characters think and feel about one another.
Best of Fiends also uses random tables to offer up background and ideas whereas Teen Detective leaves the GM with a lot of responsibility for making the game work and little in the way of structure.
Its suggested five act structure is fine as long as you're steeped in the genre and understand what makes it work. Otherwise there is no real explanation of why these stories are popular and what makes them work.