The best software program to convert a color photo to black and white is Photoshop. Photoshop's Camera Raw and Lightroom both offer powerful black and white conversion tools that allow you to precisely control the tonal values and achieve a high-quality monochrome result.
To convert a color photo to black and white in Photoshop, you can open the image in Camera Raw, then click the B&W button in the Basic panel. This will instantly convert the image to black and white, and you can then further refine the conversion by adjusting the individual color channel sliders. Alternatively, you can also use Photoshop's black and white adjustment layer to achieve a similar result, with the added flexibility of being able to fine-tune the conversion non-destructively.