From what I've seen so far, only ranking the core of the RB/HB/FB stable and from a talent perspective only..
Drake is the best option when healthy.
I suspect, even based on the limited sample size, that Ty Isaac is the second best on the roster. I wish he could work out his ball security issue and his off the field stuff.
Smith is number three. He has the heart and the will. IMO he either needs work understanding the gaps or he needs to work on his vision. The former can be a quick fix, he could be at his ceiling on the latter.
Kerridge/Houma are T4. They have done everything expected of them. They are performing exactly how I hoped a FB would under Hoke and Harbaugh
Green is last on the list but there is something to be said about the growth in his level of maturity this year. It can't be easy knowing you were primed to break out last year before an injury and now only being the 4th option. He shows more flashes of brilliance than anyone on the team but he continues to frustrate because he's so hit and miss (more miss BTW)
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Sometimes people standing on third base think they hit a triple, but they didn't - Jim Harbaugh