Greatest rookie season of all time is the great but forgotten Benny Kauff in 1914: 667 PA, .370/.447/.534, 120 R, 95 RBI, 75 SB. If you go by advanced metrics, he had an amazing .491 wOBA!!! And on top of that, he as a plus-plus defender in centerfield, giving him a total of 11.1 fWAR. Shoeless Joe Jackson is in the running too, 10.9 fWAR in his rookie season. Kauff followed up his rookie season with another monster campaign (9.6 fWAR), so I doubt there is any player in history who had such a dominating first two years. After his rookie year he only played five and a half more season though, because he was banned from baseball for allegedly being involved in an car theft ring (despite an acquittal) and for having an undesirable character. One of my favorite forgotten players! But back on-topic, if Fiers keeps pitching like this and doesn't win RoY it will be absurd. Outside Trout, the only three rookies who have been as valuable as Fiers are Miley, Darvish and Lance Lynn (all four are around two and a half wins in terms of value). But they've pitched basically twice as many innings. In his limited playing time Fiers has been the best rookie in the NL, not even close.