Thank you both for being so welcoming!
So, there are two ways to say favorite and least favorite.
The first is how well-developed they are. It is very difficult to choose, because Brandon Sanderson is remarkably good at it, so there are many well-developed characters and only a few for whom I would say there isn't enough character development for their role in the story.
I would say, I consider Dalinar the best-developed character so far. There is most visible character evolution, there is most detailed reasoning for every step forward. Also, the "weight" of all of his actions is so compelling that I can't choose anyone else.
For the least well developed in my opinion, I would go back to Elantris. I was considering several characters from there, but finally decided on Ahan. I think his background should have been much better developed to allow for the huge turn of story that he made happen.
But if the question is who are my favorites to read about, I would say Navani - as I do really enjoy the fabrial science and can easily identify with her past as well as present in the context of the story. But it is really difficult to choose, as most characters are so enjoyable! My least favorite to read about so far is Sarene. Partly because her character seems to not match the actions she is taking and there are several habits/characteristics of hers that are so pronounced, but don't have any effect on the story at all (pacing, tapping her finger on cheek, being bad at drawing, being afraid of heights etc). It's just disturbing.
I have to add that story-wise, I consider Elantris quite nice, it's just that, yeah, Brandon had not developed as a writer as far as he has now.