Who would it be?
What about Big Ten players only?
MSU's best defenses are built from the back up. So nationally, it's easy, Derwin James. He can play CB if needed, but if Scott/Layne, pan out and stay healthy, he's even more needed at safety, where he's probably even better.
Seems odd to start there, but the lack of a good secondary has crushes MSU the past two years, and I think the gap between MSU and the top guys in the nation is larger there than at QB.
But Big Ten only is trickier, because I don't see a true difference maker in the conference back there right now.
Saquon Barkley is the best player, but MSU's position with the best combination of talent and depth is RB, so I'm not sure how much better going from really good to elite gets you when you have glaring holes elsewhere.
So I'd go with Rashan Gary. You can't go wrong on the line with him, or either of the OSU guys, but I like Gary's upside more. If this is a "add to your team going forward" it gets even easier as he has to return in 2018 too.