The most effective method for converting a color photo to black and white using Photoshop is the Channel Mixer method. This method allows for precise control over the conversion process and can produce stunning black and white images. To use the Channel Mixer, go to Image > Adjustments > Channel Mixer and select Monochrome. From there, you can adjust the levels of each color channel to achieve the desired black and white result. The other methods mentioned, such as the Grayscale mode or the Desaturate tool, can also be effective but may not offer the same level of control as the Channel Mixer. It's worth noting that the best method may vary depending on the specific image and the desired outcome. Experimentation and exploration of different techniques can help you find the most effective method for your particular needs.