I think Teehan+Lax's case studies were one of the very first ones to get me excited about design as a process. I think the thing that stood out was how they took a detailed, in-depth and yet, a very holistic view of design as part of their case studies and their work. Their case studies were indeed well-crafted stories, which definitely added to the appeal and the pleasant experience of reading them. http://www.teehanlax.com

I was pretty impressed by Joel Califa's How I got hired by Digital Ocean. Eye opening for me to see how rigorous some people are.

Fi-Interactive have some of the best case studies around!

http://casestudies.f-i.com/healthcare/

I think Michael N. Evensen's case study on the SoundCloud iOS redesign is excellent http://www.michaelevensen.com/

Viget Labs do some good write ups like https://viget.com/work/puma

I love all the case studies on Haraldur Thorleifsson’s site: http://haraldurthorleifsson.com/googledrive/

I like the writings from Information Architects https://ia.net/

http://sfcd.com/work/fontspiration/ and other on this website

I love the ones Upstatement does. They're visually interesting, show you their process, and completely custom crafted to showcase the content.

http://upstatement.com/

It's a recorded presentation, but I think it's a great case study. If you can spare ~20mins, I'd definitely give it a watch.

Riot Games tackling negative online player behavior: GDC

https://medium.com/@sambrown/designing-the-new-foursquare-8f8788d366f0 is certainly among my favorite. I'm not sure it classifies as a completely legit case study but it would be cool in a portfolio in my opinion.