This completely depends on where you live in the world... In Canada our southern windows provide the best radiant heat because during the winter months the sun is lower in the sky and during mid day, also when it feels its warmest, the sun shines in your southern windows, During summer the sun is to high in the sky and does not affect our cooling applications. Eastern and western windows will provide radiant heat all year round however you need to deal with blocking the radiant heat in the summer for cooling purposes.
If your designing a home and location also consider wind direction and tree lines to block it.