* The composition: A wider shot that includes more of the environment might be better suited for landscape format, especially if the environment is important to the story of the portrait.
* The subject's pose: A horizontal pose might look awkward in a vertical format.
* The intended use: A portrait for a website banner might need a landscape format to fit the space. A portrait for a phone's lock screen is better in vertical format.
* The artistic intent: Sometimes, breaking the "rules" and using an unexpected format can create a more interesting and compelling image.
* The aspect ratio of the camera sensor or film: While most digital cameras and smartphones can shoot in multiple aspect ratios, some older film cameras may be limited to specific formats, which could influence the choice.
Ultimately, the "best" format is the one that best serves the image's purpose and the photographer's vision.