Oregon, for sure. Hypothetically speaking, we would have a month to prepare and would be much healthier than we were for the A&M game, but their offense's performance against our defense is still noteworthy. It is clear that, while still really good, our defense isn't a dominant against quality offensive opposition. And Oregon runs their offense to perfection with so many quick, talented guys out there. It would be very hard to stop. I would have (still maybe would...) really liked to see us play Oregon. It would be a very intriguing, entertaining game to watch. 50/50.
Kansas State would be a worthy opponent too, but I think we could bottle up Klein. Besides him, they seem pretty similar to us, but I think we do it better than they do. And Notre Dame would almost certainly be beat, if not soundly beaten, by any of the other three.