If I must pick, I would say The Way of Kings. I'm big fantasy fan and I was in weird place because all new fantasy books I read were kind of mediocre. But then I read Mistborn trilogy and my mind was blown. So naturally I looked up other Sanderson books. To be honest, I didn't expect much, since some authors become predictable or use the same magic system but Elantris and Warbreaker were different from Mistborn. And then, just before reading WoK, I learned about Cosmere so when I started reading I knew that I could expect something really good and I was not disappointed. Also, I really like big books.
Plus, since english is not my first language, some english fantasy books are sometimes really annoying with (pretentious) writing style, for example I have filling that some writers use more complicated sentences and words to describe epic or medieval worlds and Sanderson doesn't do that. He can so masterfully describe magic and different worlds using simple language. And I noticed that reading WoK.
Hmm that's really hard question because my thoughts about characters sometimes change with rereads.
That being said, I really liked Vivenna and Nightblood so I was excited to see them in SA. I hope we will see more of them.
Also, I didn't like Rsyn interludes in the beginning but she changed as a character and I started to really like her (and I appreciate the character she was in the beginning). After Dawnshard, I'm more sure than ever that she is one of my favorites.