In terms of winning percentage it would have to be Day first (after Woody of course. Woody will always be #1.
Anyway, in the entire 150 yr college football annals, Ryan Day is the third winningest coach by percentage, after the guy who started it Walter Camp, and Knute Rockne. I was surprised to hear that, but then several sites/anchors/pundits said the same thing. Yep, he has a higher winning percentage than Nick Saban.
Nick is 13th in percentage but one in number of wins. This includes his entire career, not just Alabama.