Have to agree with thewupk that it's perception more than production that would have you label Donaldson "one notch below the best". As I posted a few days ago, since debuting, Donaldson's been second only to Trout in position-player WAR—and he's been significantly better than "superstar" Bryce Harper. For fun, over Harper's career:
(*Jose Altuve)
2012–2017 Seasons Player bWAR fWAR Mike Trout 54.5 53.7 Josh Donaldson 37.6 37.1 Abies' Future* 28.9 26.0 Manny Machado 27.9 26.0 Bryce Harper 26.1 27.7 Jason Heyward 26.0 23.6 Freddie Freeman 25.2 24.9
And I chose Harper's entire six seasons in the big leagues, as opposed to the most-recent-five I originally had in mind, because including 2012 stats actually benefits Harper somewhat substantially.
Obviously, bWAR and fWAR don't tell the whole story, but they tell a lot of it; and the fact is, people (very wrongly) try to talk about Harper in the same breath as Trout all the time, but Donaldson's been a lot closer to that echelon than the 26-year-old "superstar". Indeed, if either of the two belong "one notch below"—for now, based on actual production—it's Bryce Harper, who's heretofore had a career only slightly more productive than Jason Heyward or Freddie Freeman.