It would probably be Iowa this year for me. ISU hasn't beat Iowa in a while, it's at Iowa, and there are fewer big dogs (not really any) on the schedule anymore in OU or UT to make up some good will and boost our rankings. Utah or K State or someone else may end up being more important, but as we sit right now, winning at Iowa seems most important for ISU this year.
Also for the credibility of the conference, I hope ISU beats UI, WVU beats Pitt and Penn State (you've got two good opportunities at big non-con wins!) Houston wins at Oklahoma, Okie State beats Arkansas, Kansas beats Illinois, and ASU beats Mississippi State at home. Those appear to be the biggest games I see for the conference after a quick look at the noncon. Maybe throw CU winning at NU on there, but we know NU's not great lately. Still, it would be a road win over a BIG opponent. Not a lot of great opportunities at resume builders beyond those, so hopefully the conference can win these games to set us up for getting two into the CFP and building more respect to carry through the whole remainder of the year.