i am looking for a filament with the hardest surface finish. Any ideas?
Nylon (e.g. Taulman 645) gives you parts that have an extremely tough surface. They are almost impossible to sand or file. But nylon is a little flexible, so it depends on what you are looking for. You also have to keep nylon dry before printing.
If it's just about the surface, then stainless-steel filled PLA will be hard to scratch - the steel particles will protect the PLA to a degree. If you're looking for a super stiff filament, then filaments filled with carbon fiber will do, starting with nylon, then polycarbonate and if you have a really big wallet, then carbon-filled PEKK is the ultimate filament in most aspects, including hardness/stiffness.
Voj: Which of the PEKK's have you printed with? And any photos of the final surface quality?
Check out this paper, it is normally used for supports due to its chemical properties but depending on the loading scenario HIPS could be a good option. https://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=1026&context=aseeil-insectionconference