What was the single greatest day a player scored in fantasy football? It is an interesting question, and unfortunately, due to incomplete box scores from times of yore, it might not be answerable.
After scouring through the data at Pro Football Reference, here's what I found:
|Billy Cannon||HOU||Dec 10, 1961||216||3||113||2||0||63.0 pts|
|Gale Sayers||CHI||Dec 12, 1965||113||4||89||1||1||56.2 pts|
|Dub Jones||CLE||Nov 25, 1951||116||4||80||2||0||55.6 pts|
|Clinton Portis||DEN||Dec 7, 2003||218||5||33||0||0||55.1 pts|
|Cookie Gilchrist||BUF||Dec 8, 1963||243||5||0||0||0||54.3 pts|
|Abner Haynes||DAL||Nov 26, 1961||158||4||84||1||0||54.2 pts|
|Cloyce Box||DET||Dec 3, 1950||0||0||302||4||0||54.2 pts|
|Jim Brown||CLE||Nov 19, 1961||237||4||52||0||0||52.9 pts|
|Shaun Alexander||SEA||Sept 29, 2002||139||4||92||1||0||52.9 pts|
|Jerry Rice||SF||Oct 14, 1990||0||0||225||5||0||52.5 pts|
|Jamaal Charles||KC||Dec 15, 2013||20||1||195||4||0||51.5 pts|
|Doug Martin||TB||Nov 4, 2012||251||4||21||0||0||51.2 pts|
|Jerry Butler||BUF||Sep 23, 1979||12||0||255||4||0||50.7 pts|
|Mike Anderson||DEN||Dec 3, 2000||251||4||5||0||0||49.6 pts|
|Adrian Peterson||MIN||Nov 4, 2007||296||3||19||0||0||49.5 pts|
And possibly the greatest fantasy day in NFL history is incomplete.
Ernie Nevers (RB, K - Chicago Cardinals) on Nov 28th, 1929 vs. Chicago Bears rushed for 6 TDs and kicked 4 extra points. He missed two. Unfortunately, I can not find his total yards, but including the 42 yards just from his touchdowns, gives a minimum 44.2 pts
Read more about the Ernie Nevers story from the Chicago Tribune article: Nevers Again.
Scoring Note: 6 pts per touchdown, 1 pt per 10 yds rushing, 1 pt per 10 yds receiving. Most leagues have scoring for QBs, giving only 1 pt for 25 yds passing, and 4 pts a TD, so, for example, Roethlisberger who threw for 6 TDs and 522 yds would only net 44.9 pts for that monster day.