If you played through the entire game and didn't find any of the stories compelling, all I can really do is shrug and say apparently it wasn't the game for you. I'm curious what games you've felt were "non-flat" and well written?
If you do want examples of great stories from Witcher 3, they are (IMO):
* Main story from the Hearts of Stone expansion
* Bloody Baron sub-plot
* Main story from the Blood & Wine expansion
* Triss Merigold sub-plot regarding mages in Novigrad
* Haunted tower quest from Kiera Metz
* The first 75% or so of the base game's main story is good, but it's complex and hard to get invested in if you aren't familiar the characters and backstory
If you do want examples of great stories from Witcher 3, they are (IMO):
* Main story from the Hearts of Stone expansion
* Bloody Baron sub-plot
* Main story from the Blood & Wine expansion
* Triss Merigold sub-plot regarding mages in Novigrad
* Haunted tower quest from Kiera Metz
* The first 75% or so of the base game's main story is good, but it's complex and hard to get invested in if you aren't familiar the characters and backstory