Masashi Kishimoto has gone o record to say that among the Naruto main cast, Sasuke Uchiha is his favorite character. That's easy enough to see by looking at the Naruto narrative; he goes from a basic rival character to the deuteragonist of the series; his abilities and lineage both become key plot points; several story arcs in Part II are devoted to him. If Sasuke wasn't Kishimoto Sensei's favorite, he'd have to at least be close to it.
What's really difficult to imagine, however, is why Sasuke is his favorite character. For all the reasons there are to like a character, this is something only he and a select few fans can truly understand. It doesn't have to do with his personality or anything he does in the story. Getting to the bottom of this will require a look at Sasuke's creation and conception.
The funny thing is that Kishimoto Sensei never even considered having Sasuke in his story at first. His editor was the one who convinced him to give Naruto a rival. If Kishimoto never even wanted Sasuke in his story to begin with, then that only adds to the mystery of what changed.
Not only did Kishimoto Sensei not have Sasuke in mind when conceiving Naruto, but he had a difficult time just drawing the character. His original design for Sasuke featured several necklaces and ties around his arms and legs. He had to scrap this design because he couldn't draw it on a weekly basis; he instead went with a more streamlined look that would contrast Naruto's outfit.
Kishimoto Sensei would try again to give Sasuke a cool design during the Chūnin Exams. He gave the boy several belts around his left arm and bands around both his legs. Unfortunately, like Sasuke's beta design, it was unsustainable, so Kishimoto Sensei eventually switched him back to his old design.
Kishimoto Sensei couldn't even get Sasuke's face right at first. He needed to make the boy look like a kid in Part I, but he would always make him look older than he really was. It would take him some time to settle on some relatively boyish features and stick with them, and even then he struggled to maintain them in his art. He probably felt so relieved after making everyone into teenagers for Part II.
For all the difficulty drawing Sasuke gives Kishimoto Sensei, one would think that he'd be the mangaka's least favorite character. However, he calls Sasuke his favorite precisely because he gets so much enjoyment out of drawing him. He likes coming up with cool new designs for a cool character. It may sound tedious, but, for a true artist, it's a lot of fun.
This would certainly explain Sasuke's design changes so often throughout Part II. Not only that, but his hair gets a little longer, which also means he takes a little longer to draw. Even Naruto only gets an occasional outfit change, which always inevitably reverts back to his standard wear. Sasuke, on the other hand, is ever-changing and never goes back to an old look.
Not only is it obvious how much Kishimoto Sensei loves Sasuke, but it's also clear how much he loves drawing him. As difficult as it can be, there's a pleasure to be had from dressing up characters in new clothes every once in a while. It's a feeling that can be understood well, especially among fellow artists. Kishimoto's passion for designing and redesigning Sasuke is indicative of his passion as an artist.
