I've done this the past two years; might as well continue.
Here are the rankings I've come up with. For every team a team beats, it gets 1 point for each defeated team's victories (adjustments for wins over FCS schools); for every team it loses to, deduct one point for each victorious team's losses (again, adjustments for losses to FCS schools). Divide by the number of games played. The results:
1. Alabama 6.29
2. Ohio State 5.79
3. Clemson 5.50
4. Western Michigan 4.54
5. Michigan 4.21
6. Washington 4.13
7. Penn State 4.04
8. Colorado 3.83
9. Florida State 3.75
10. Navy 3.73
To really make this work, I need to go another iteration, but I'd have to figure this for all FBS teams, and that would take either a computer or time even this $9/hour worker doesn't have.