Well I based this on my projections for them... here are my projections...
Darren McFadden - 900 rushing yds, 6 rush TD, 40 receptions, 385 receiving yards, 2 rec TD
Knowshon Moreno - 1025 rushing yds, 7 rush TD, 30 receptions, 250 receiving yards, 1 rec TD
Derrick Ward - 950 rushing yds, 5 rush TD, 38 receptions, 315 receiving yards, 1 rec TD
Larry Johnson - 1025 rushing yds, 7 rush TD, 20 receptions, 160 receiving yards 0 rec TD
Jonathan Stewart - 850 rushing yds, 9 rush TD, 13 receptions, 80 receiving yards, 0 rec TD
Are those unrealistic projections? Definitely not. But based on those, you see why I ranked them the way I did. All very close in projected fantasy points, so obviously everyone's opinion will differ. I only see the top 4 varying in overall value, but Stewart should finish at the bottom of that group.
That about sums it up there. Everyone's opinions will differ.
Are they unrealistic? nope. Any of these backs could end up with 800 total yards and 4 TDs, and any of these backs could also end up with 1400 total yards and 14 TDs. That's just what you're looking at in this point in the draft. I'm expecting around 1300 yd, 30-40 rec, and 10-12 TD for LJ. That's only 115 yd, ~15 rec, ~4 TD away from you. They got a new offense, a better QB, and he appears to be happy. However his body could be done. 900 yd, 15 rec, 4 TD could be all he has left.
When i'm making my rankings, i prefer to go by opportunity and talent. I have a very hard time ranking any backup RB ahead of someone who will be getting 70% of the carries. There is just so much more risk involved when a players value is dependent on another players performance, and they have very little control over it. Not only is Ward going into a new offense with a brand new head coach, if Graham comes out of the gate with 2-3 100 yard games, Ward's value will plummet. There are rare exceptions though. In a PPR league you may have a Reggie Bush who will get 60, 70+ receptions. Or if you play in a league with return yards, Darren Sproles not only will be 2nd on the team in carries, and get 20, 30+ receptions, but he'll be racking up plenty of yardage and a couple TD via special teams.
My reasoning for ranking Stewart over DMC is simple. though DMC is the "starter", I expect Stewart to get the same number of carries, so their opportunity is basically the same.
It's still early though. If Ward takes over as the starting RB in TB, i'd bump him from 5th to 3rd.