Where is the best place for a bird house?

There is no general "best" place for bird houses. It depends on the birds you are attempting to attract. Some birds will use bird houses that are in trees, and some will not. In the long run, unless you specifically want to attract a certain bird, you should place the bird house where you will see it and enjoy it most. You might want to research some websites or visit your local wild bird store to get their ideas, because it is too complex to cover all birds in the world here. See the link below for a start.