So there are three main ways to objectively compare coaches. These methods work across conferences, and through time.
First way is total number of lifetime victories. Sticking with FBS, that list goes like this:
1. Joe Paterno, 409
2. Bobby Bowden, 377 (389 on the field, but 12 were vacated)
3. Bear Bryant, 323
4. Pop Warner, 319
5. Nick Saban, 292
[there are some famous non-FBS coaches who would be high on that list, like John Gagliardi, at an astounding 489 victories, Eddie Robinson (408) and Amos Alonzo Stagg (314), if you're into all tiers of college football]
Second way is winning percentage. That list looks like:
1. Knute Rockne, .881
2. Frank Leahy, .864
3. Urban Meyer, .854
4. George Woodruff, .846
5. Barry Switzer, .837
[Nick Saban is at #14 on that list (.804), five spots lower than General Robert Neyland (.829)]
Finally, one can count national championships (conference championships would be useful comparing coaches inside a conference, but lose utility nationwide). Here is where Saban has a claim as GOAT:
1. Nick Saban, 7
2. Bear Bryant, 6
3t. Bernie Bierman, 5
3t. Woody Hayes, 5
5t. Robert Neyland, 4
5t. Frank Leahy, 4
5t. John McKay, 4
Is Saban the best ever? As you can see above, it's arguable. He's definitely ONE of the greatest.
Go Vols!