IMO, there's a clear top two. Trey Sermon and Journey Brown.
Last season Trey Sermon ran the ball 54 times for 385 yards (7.1 avg), and caught 8 passes for 71 yards. Part of that was due to injury, and part of it was due to the emergence of two other backs. In addition, Jalen Hurts ran the ball 233 times for almost 1,300 yards.
Diving deeper into Sermon's stats:
Of the 385 rushing yards Sermon had, 91 came against the #85th ranked rushing defense (Houston), 76 came against the #80 ranked rushing defense (Texas Tech), 71 came against the #124th ranked rushing defense (Kansas), 56 came against an FCS opponent (South Dakota), and 51 came against the #52 ranked rushing defense (UCLA).
The combination of these factors leads to a very unimpressive year for Sermon, and probably contribute heavily to him leaving OU.
Let's look at his 2018 numbers, because I believe these to be more reflective of how Sermon will perform at OSU.
He rushed 155 times for 928 yards (6.0 avg), and caught 12 passes for 181 yards.
206 of those yards came against Texas Tech, the 65th rushing ranked defense.
124 came against Oklahoma State, the 85th ranked rushing defense
119 came against Army, the 4th ranked defense.
and 110 came against TCU, the #38 ranked defense.
Sermon had a much better year in 2018.
IMO, even if Sermon returns to his 2018 form, he isn't as good as Journey Brown. However, he will be running behind a very good offensive line, and I really can't argue too much if people rank him #1.
