Source Media
For the highest possible scan quality, obtain the best, most original copy of the image you can find:
1. A negative or slide is best because it is the original1), has the best possible image quality, and is not cropped like a print. Send these out to a slide/negative scanning service for best results.
2. A photographic print made from the original negative or slide is second best, because it's a copy of the original.
If you have prints of various sizes of the same photo, then scan the largest one. If a smaller print has better color or fewer flaws or different framing (for example, a 3½×5 photo is wider and an 8×10 is taller), scan that one also. If you have a negative and also a print, scan both just in case.
3. A copy of a photographic print is third best (that means worst) because it's a copy of a copy. Scan it now, but keep searching for #1 or #2.
If you don't have the ideal source media, scan what you have now while you wait for something better. Your photographs will not improve with age, they can only degrade.
If you ever rescan a photo, perhaps because you found a better copy to scan, do not delete the older scan unless you are sure that the newer scan is as good or better than the old scan in every way.