Despite its amazing strength titanium is actually pretty soft. Stainless steel? I do not believe I have ever seen a stainless steel light. I have plenty of knives with stainless steel blades of many different high end very expensive types. Despite being very hard they still scratch and left out in the weather will rust. There is no truly 100% stainless steel.
Aluminum. There are more types and grades of aluminum than I can count. Most modern lights are of "aircraft grade aluminum" which is supposed to be light very strong and damage resistant. Still will scratch fairly easy though. Seen some copper lights. The 2 I have handled just would not scratch or mark up. Makes no sense as copper is actually pretty soft.
Most scratch and damage resistant lights I have are my 3 Gen3 Armytek lights. They are aircraft grade aluminum but have the thickest toughest anodization I have ever seen on any light ever.
So the answer is basically a light made of actual "aircraft grade aluminum" with the most properly applied HAIII coating. I have noticed thickness differences in the anodization between some of my lights. Armytek is the thickest by far its very grippy. Anytime I have dropped one or used it roughly to where a mark appears the mark rubs off quick and easily. Jetbeam-the aluminum on my 3 Jetbeams must be an eighth of an inch thick. The anodization on all 3 is perfectly applied and thicker than average. I have yet to leave any kind of scratch or mark on any of them that can be seen with the naked eye.
Mohs hardness scale is a measure of scratch resistance...tungsten carbide has a rating of 9, but would be very heavy.