Trump, by less than a point.
Tilt D. Brown won by 12 in 2018 while Cordray lost by 0.1%.
I'd say Biden by 3 or 4 points.
Cordray lost to DeWine by 4% in 2018, while Clinton lost to Trump by 8%. I would say that Montgomery County is probably a Tossup, but Biden could very well flip it, even though Trump will carry Ohio as a whole again.
Important to keep in mind that while Biden is likely to do worse than Cordray did in 2018, this particular county's democratic base consists of many low-propensity African American voters who only turn out in presidential years. With that being said, Cordray probably won some Trump voters that will stick with him, so tossup seems appropriate.
Cordray probably lost it in big parts because DeWine is from the Dayton Area.