I'm not sure why but this topic gets pretty passionately dicussed sometimes. You might try searching the forum for past threads.
Bottom line, imo, is that there isn't one finite answer. I think whatever works for each pup is what's good for THAT pup
Mandie has always done well on grain free so I keep her onBlue Buffalo.
Just remember to gradually transition any food change to keep upset tummy at bay. You may just need to try various ones until you find what works.