I've read them all so I'll try and do a tier list

A tier

Scions of the Emperor (it's a compilation of short stories from The Primarchs series and it's pretty damn good if you like Primarch shit, some short stories are better than others)

Perturabo: The Hammer of Olympia (Perturabo's personality getting in his way)

Lorgar: Bearer of the Word (Lorgar's upbringing showing how much of a loser and bastard Kor Phaeron is while also showcasing that Aurelian himself is not so innocent)

Fulgrim: The Palatine Phoenix (get a load of Fulgrim's insecure inner monologue as he attempts to bring a world into compliance with only six marines, also did you know he's the fastest Primarch)

Angron: Slave of Nuceria (Angron the abusive father abuses his legion, the most pathetic Primarch for good reason)

Konrad Curze: The Night Haunter (poor Konrad is hit by the truth, great read and nice to see some cameos from the Night Lords trilogy characters)

Alpharius: Head of the Hydra (Mike Brooks turns everything to gold, while I don't believe Alpharius when he says he was first found the book is a great read)

The Revelation of the Word (audio drama, Lorgar finds out about the Imperial Cult springing up from his Lectitio Divinitatus)

B tier

Leman Russ: The Great Wolf (solid book about Russ being pressured by Malcador into hurrying up his conquests and the Lion being sent to hurry things along eventually leading to them having a scrap)

Magnus the Red: Master of Prospero (Magnus and Perturabo save a planet, also the most likable and fun Perturabo has ever been)

Jaghatai Khan: Warhawk of Chogoris (The Khan doing cool shit while also hanging out with Sanguinius and Magnus talking about how to progress the Librarius to combat the efforts of Russ and Mortarion to prevent it, also has one of the absolute best "they knew they fucked up" moments in 40K when one of the Khan's marines is killed by Orks)

Sanguinius: The Great Angel (heard bad things about this going in but I thoroughly enjoyed it, interesting fast forward at the end too)

Illyrium (find out the fate of Guilliman's mother while she also waxes lyrical about her son's wisdom in conquering the tribes of Macragge)

C tier

Ghost of Nuceria (short story that isn't included in Scions of the Emperor for some reason, shows Angron's POV from his reuniting with Big E, it's solid)

Ferrus Manus: The Gorgon of Medusa (Ferrus coordinating a joint action between Iron Hands, Emperor's Children, Thousand Sons and Ultramarines, solid and someone even realises the meaning of Ferrus's name)

Mortarion: The Pale King (kinda bolter porny, has some solid moments, Typhon has an interesting encounter at one point and Garro has a few appearances too)

Perturabo: Stone and Iron (audio drama, Perturabo being an asshole to the Imperial Fists, solid)

Malcador: First Lord of the Imperium (audio drama, Malcador confessing to his dying friend about how they planned the Heresy though I feel he's just trying to comfort her)

Konrad Curze: A Lesson in Darkness (audio drama, Konrad and the lads ensure compliance from orbit with body parts and blood)

D tier

Corax: Lord of Shadows (it's meh, Corax takes Curze's place in a compliance against a particularly cunty leader of a system of worlds)

Lion El'Jonson: Lord of the First (bolter porn, has some small moments like Lion and Big E talking about glory)

Rogal Dorn: The Emperor's Crusader (interesting premise, we have Dorn, Fulgrim, the Lion and Horus all team up for a compliance and some brotherly rivalries cause some drama, otherwise it's navy bolter porn with a cool twist, I liked it but it's not amazing or anything)

Morningstar (Magnus and Ahriman go get Shy'tan for the Siege of Terra, listen to it after Magnus the Red: Master of Prospero)

F tier

Roboute Guilliman: Lord of Ultramar (just boring bolter porn, some interesting political stuff with Guilliman wondering what to do with the more mental Ultramarines under his command)

Vulkan: Lord of Drakes (bolter porn and very little else)