What is the best way to clean a scratched CD?

There are mant items that can be bought commercially to do such a thing. They can range from cheaper to way expensive and professional grade.

A home remedy (no responsibility here) is to use soft non gritted toothpaste lightly with a lint free cloth to wipe it and buff it lightly. Then clean CD with window cleaner or lens cleaner.

You can try to take it to a blockbuster, family video, or any video store with a professional cleaning tool / machine and ask them to clean and repair the CD/DVD also.

Hope this helps