It's opinion based, there is not definitive answer to that question.
Sakura surely is the best medical ninja in Konaha. It is even stated that she surpassed Shizune making her only second to Tsunade.
Interesting post for further readings, from where I got the images
As for kabuto, he is one of the greatest medical ninja that we’ve seen in Naruto. He was easily on par with Kakashi in Part 1, and after absorbing a DNA from various people in part 2, he became a lot stronger. He possessed amazing healing abilities, for which Orochimaru chose him as his student. Furthermore, Kabuto possessed the healing abilities of the great white snake, and also Karin, along with his own medical skills.
But if I had to say who's the best between both of them I would have to say Kabuto, he seems really talented and even Tsunade was surprised at how skilled he was and even said that Kabuto's "skill" (sharpness and jutsu sense) surpass those of hers at her prime. Sakura has shown alot of progress, but I can't really call her the best since we can't tell how much stronger she will get. As of right now my choice is Kabuto.