Tuitt would easily be first. Not even close, 22 sacks speak for itself. Tillery has already accomplished more in his FR. year than Nix did his FR. year. KLM was a solid player and team leader. Rochell is under appreciated by many. Really good player. Day would be 2nd behind Tuitt.
Kap had 175 tackles in his ND career, Tuitt had 124. Tuitt had a year of limited rotational duty, 2/3's of a season where he looked like the best defensive lineman in the country, and one decent season his Junior year. I'd personally take 2012 Kap over 2013 Tuitt. Kap was a four year starter, and had three seasons of very good play, and 2012 was just a bit short of elite. Tuitt had 8.5 sacks in the first seven games of 2012, and 11.5 in his next 19 games. We really only saw fewer than 10 games where Tuitt played at an elite level his entire ND career. Kap played basically at a good to very good level for basically four years. Just depends on what you value more I suppose.
Nix IMO was the biggest impact player on the defense in the 2012 season (which I know a lot of people would argue against, to each their own). A huge part of how stout we were in the red zone in 2012 was that running up the gut with Nix there (along with a great cast admittedly) was a loosing proposition every time.
Day would be right up there if he were healthy for more of his career, and if he keeps up the pace he's been at he can certainly make up for a lot of that.
We could discuss this all day, the D-line certainly all got hot at the same time in 2012. We haven't had a healthy, championship caliber playing together since. Still an underrated unit, but needs more elite blood in there.