My out of state investors have chosen Toledo because of its superb stability as a rental market. Over half of our city rents and it has been like that for years. The rent to price ratio is better than the majority of the country as well. In regards to population decline, as it is certainly true, you must remember that there is still demand for housing in submarkets of each market. For an example, in Toledo, no one wants to move into the 43608 zip code. Most houses in that area are vacant and will be torn down in the distant future, but in regions like 43623 near the mall, we have seen significant growth and demand for housing, especially in multifamily.
I think Ohio is a great place to park your money. There will always be cash flow investors buying here, so you have an exit strategy if your main concern is property value decline. I think it's also a forgiving market for the out of state investor because the cash flow is so strong that you get a great taste of what it is like to invest from afar without worrying about losing money.
If population growth is your main concern I would go to Columbus.
If cash flow is your priority, I would choose Toledo or Cleveland.
Happy to help if you need it :)