Best placement is typically dead center in your home on the top floor away from any metal items/air ducts.
Second best is usually the basement in the center of the home, away from all metal items as possible.
If you haven't noticed a pattern to the about, you will note the center of the home and away from metal items. Metal items will block the signals (so if you can't picture a direct line from where you want to have a god wifi signal and the wifi router without it going through something that is metal such as a refrigerator, TV, washer/dryer, air ducts, etc., then you need to consider changing the location).
Typically I prefer the basement if you have neighbors that are nearby as this will help limit the distance and locations that others can attempt to be located to hack into your network. But if you are trying to get as much coverage as possible to reach a large backyard, etc., then the top floor is usually the best location due to typical signal field shape from most antenna used in wifi routers..