Indiana. Tennessee was a looooong time ago, and given he has passed on the 2018 race has two incumbent Republicans. Colorado is both left-leaning and weaker ties to Manning.
Indiana has a democratic incumbent (though I guess he's passed on that race as well) and is by far the state where Manning would have the most appeal. He might be the one Republican who could win Indianapolis lol
Doesn't matter though, he has no interest in politics from what I can tell. Probably doesn't want to become a divisive figure.
Peyton Manning was an excellent player and leader for the Tennessee Titans. When he left, the team stopped winning consistently.
He played for the UT Vols in college; he did not play for the Titans. Even though it was long ago, he's beloved here; he could win the Senate pretty easily.