What is the Best Material for Chrome Guitar Pick Guard?
I'm looking to cut a piece of metal in the shape of a pickguard, then have it professionally chromed

The chrome will go away quite fast, considering the abuse it will have to take - regardless how kind and attentive you are while playing the guitar (also depending how much time you play that guitar, and the material and quality of the pick itself). Unless you &quot;professionally chrome&quot; that metal every few months / years, the base metal will be there smiling at you all the time.
My best advice is to use stainless steel - it is metal, it is strong, it looks like chrome and it does not need any chroming. It might be more expensive for the beginning, but it will need zero maintenance afterwards.

A cheaper alternative is to use copper or aluminum. They are easier to find (I saw aluminum sheets myself quite often in generic home-repairs and gardening shops), easy to cut and polish, easy to chrome. Aluminum also is very light, it will not break the back of the player with the extra kilograms. Brass also comes to mind, as a base material.