The question about reading order often comes up, and honestly, there are a couple of valid paths, but I think publication order is the most rewarding. Start with the Shadow and Bone trilogy: 'Shadow and Bone', 'Siege and Storm', then 'Ruin and Rising'. That's the core Alina Starkov story.
After that, you can move into the Six of Crows duology: 'Six of Crows' and 'Crooked Kingdom'. These books are set in the same world but feature a mostly new cast of characters in Ketterdam, and they are, in my opinion, where Bardugo's writing really hits its stride. The plot is tighter and the ensemble is phenomenal.
Finally, tackle the King of Scars duology: 'King of Scars' and 'Rule of Wolves'. These books bring characters from both prior series together. Reading them last is essential because they heavily spoil events from both the original trilogy and the Crows books. Jumping here first would ruin so many surprises. Some suggest reading the Crows first since they're better, but you lose the context of the Grisha world and the fallout from the trilogy's events that shape the political landscape the Crows operate in.
Chronologically, the Crows duology happens after the Shadow and Bone trilogy, but before King of Scars. Publication order respects the author's unfolding of the world and the natural progression of spoilers.