DonAK
President of FC Barcelona
imo Khedira is still better than Pjanic so it kinda includes that he's also beating Rakitic there aswell. Guy is an undoubted starter at Juventus and the german nationalteam, keeping a fit Gündogan and Goretzka on the bench. He did the Casemiro-role at Madrid, before Madrid realized they need a guy doing the Casemiro role. Any midfield he was in was functional, which indicates that he's doing a fantastic job as a whole.
Busquets obviously is the best of all them, but both pairings are imo comparable.
Apples and Oranges.
Pjanic cannot do what Khedira does. Rakitic can to a certain degree. Rakitic's best performances in his career have been as an AM/B2B, almost like a CAM at times like in Sevilla. Neither Pjanic or Khedira plays that position, and unfortunately for Rakitic he will never either in Barca.
Pjanic's style of play and skillset is the most suited to Barca out of all three if we start focusing more on the midfield instead of bypassing it or using a part of it as a workhorse. However Pjanic is also someone who disappears in bigger games which have been his biggest point of criticism throughout his career.