So, if I go with the category of greatest "Career" NFL running backs. Emmitt is easily in my top 5 and perhaps #1 all-time
Regardless of his O-line or offensive system
Obviously, a great back such as Sayers had his career cut short and couldn't compare, along with possibly Sanders who retired just past his prime.
The list of single season rushing yards
1 Eric Dickerson+ (24) 2,105 1984 RAM
2 Adrian Peterson (27) 2,097 2012 MIN
3 Jamal Lewis (24) 2,066 2003 BAL
4 Barry Sanders+ (29) 2,053 1997 DET
5 Terrell Davis+ (25) 2,008 1998 DEN
6 Chris Johnson (23) 2,006 2009 TEN
7 O.J. Simpson+ (26) 2,003 1973 BUF
8 Earl Campbell+ (25) 1,934 1980 HOU
9 Ahman Green (26) 1,883 2003 GNB
Barry Sanders+ (26) 1,883 1994 DET
11 Shaun Alexander (28) 1,880 2005 SEA
12 Jim Brown+ (27) 1,863 1963 CLE
13 Tiki Barber (30) 1,860 2005 NYG
14 Ricky Williams (25) 1,853 2002 MIA
15 Walter Payton+ (23) 1,852 1977 CHI
16 Jamal Anderson (25) 1,846 1998 ATL
17 DeMarco Murray (26) 1,845 2014 DAL
18 Eric Dickerson+ (26) 1,821 1986 RAM
19 O.J. Simpson+ (28) 1,817 1975 BUF
20 LaDainian Tomlinson+ (27) 1,815 2006 SDG
21 Eric Dickerson+ (23) 1,808 1983 RAM
22 Larry Johnson (26) 1,789 2006 KAN
23 Emmitt Smith+ (26) 1,773 1995 DAL
24 Adrian Peterson (23) 1,760 2008 MIN
25 Marcus Allen+ (25) 1,759 1985 RAI
26 Terrell Davis+ (24) 1,750 1997 DEN
Larry Johnson (25) 1,750 2005 KAN