From ESPN Stats & Information, the longest average home run distance in the National League (minimum 15 home runs):
Bryce Harper, Nationals: 414.1 feet
Justin Upton, Diamondbacks: 413.8 feet
Carlos Gonzalez, Rockies: 413.7 feet
Wilin Rosario, Rockies: 413.3 feet
Harper's raw power isn't a surprise, of course, although it's still impressive that he edges out guys who play in Arizona and Colorado. D.C. might have plenty of hot air, but not the thin air of those places.
As for the NL Rookie of the Year, I think Harper is now the clear leader, and not just based on his publicity advantage over other top candidates. Since Aug. 17 he's hit .301/.358/.596, with nine home runs and 19 extra-base hits in 136 at-bats, raising his season OPS by 63 points. Reds fans will advocate for Todd Frazier, who does have a higher OPS on the season, but the comparison between the two isn't that close. Frazier has been a platoon guy at times, and while Frazier was superb filling in on a regular basis for Joey Votto, Harper has scored 92 runs while playing very well in center field on an everyday basis.
My No. 2 guy is actually Diamondbacks left-hander Wade Miley, 16-10 with a 3.25 ERA, pitching in a tough home park.
NL rookie WAR leaders (via Baseball-Reference):
Norichika Aoki, Brewers: 3.0
Zack Cozart, Reds: 2.7
Lucas Harrell, Astros: 2.6
Andrelton Simmons, Braves: 2.5
Mike Fiers, Brewers: 1.9
Aoki's solid season has gone unnoticed, but he was a big reason the Brewers clawed back into the wild-card race. Miley has been terrific. But Harper is the Rookie of the Year in the NL.