I'm a little biased... ok, maybe a lot of biased, but I've got Walter Payton #1.
But I can see the argument for Jim Brown and Barry Sanders as well.
My list is broken down kinda like this:
Group 1: The All-Time Greats
1- Payton
2- Brown
3- Sanders
Group 2 - The Greats
4 - OJ Simpson
5 - Emmit Smith
6 - Faulk
7 - Dickerson
Group 3: The "Wish they stayed healthy" - they coulda jumped into group 1 with longer/healthier careers
8 - Earl Campbell
9 - Gale Sayers
it feels like I'm forgetting a person or 2 as well, b/c I used to have a full top 10. Basically I feel guys are interchangeable in their groups, but can't exceed the top ranking in their group