Any single stat in isolation is pretty irrelevant, imo, so it's hard to pick one that is most so. Just look at total offense, for example. Texas Tech led the nation in total offense with 567 ypg and they went 5-7. Total defense looks like a better barometer with Alabama and Michigan #1 and #2 as well as a slew of other good teams in the top 10. But BC lost 6 games with the #9 ranked defense in the land.
As far as offensive stats go, I'd like to see how yards on first down correlates to scoring and offensive efficiency. (I was going to say how it relates to wins and losses, but that depends heavily on D and ST's.) So here's what I found out:
Clemson led the nation in first down offense. Shocker. In addition, a lot of really good scoring offenses were also high in this stat. Below I listed the top ten teams in first down offense and where they ranked in scoring in parenthesis.
1. Clemson (14)
2. Tulsa (6)
3. Texas Tech (5)
4. La Tech (2)
5. W. Mich (9)
6. W. Kentucky (1)
7. Wash. St (18)
8. Cal (22)
9. Va Tech (33)
10. Oklahoma (3)
After spending a good bit of time looking at a bunch of stats individually and how they were able to predict final rankings, I couldn't find one that stood out above the rest. It really is a mix of stats that determines success and not one more than any other. Maybe scoring defense if I had to pick.