Regular Season
As listed on Royals website.
http://kansascity.royals.mlb.com/kc/ballpark/history.jsp
Sept. 14, 1986 - Bo Jackson launches the longest home run in Royals Stadium history, a 475-foot shot off Seattle's Mike Moore. It was Jackson's first career home run.
All-Time
As listed on Royals website
http://kansascity.royals.mlb.com/news/gameday_recap.jsp?ymd=20060808&content_id=1599978&vkey=recap&fext=.jsp&c_id=kc
July 24, 1973 - Catcher Johnny Bench of the Cincinnati Reds hits a 480 foot home run during the 1973 MLB All Star game at Kauffmann Stadium in the 4th inning.
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Mickey Mantle. Check out the link for his ten longest home runs. http://www.themick.com/10homers.html
Harmon Killebrew was known for his long distance power hitting. His longest measured home run was 520 at Minnesota's Metropolitan Stadium. He likely had longer ones that were not measured.
Here is a post listing the Longest Home Runs ever hit by a Mets player at Shea Stadium http://metsblog.com/metsblog/top-5-longest-home-runs-by-a-met-in-flushing/ Dave Kingman & Mo Vaughn are the winners
Ted William's longest home run in Fenway Park was 502 feet, but I don't know if that was his longest home run ever.
Josh Hamilton 502 feet to the upper deck of home run porch (right field)