Which Dremel bit for removing material from pistons?

There are quite a few different Dremel bits that are for carving etc and comes with different sizes, materials etc.

Which one of those is a "perfect" for removing material form pistons?

There isn't any "perfect" tool as, depending on where you want to remove the material, the tool shape will change. A large diameter HSS ball, or parallel cutter like the 114, 134, or 115 would probably be my choice if I needed to remove a lot of material, with smaller versions if I needed to do some finer, more detailed work. Aluminium can be quite 'gummy', and stick to the cutters - some people use candle wax, or other products that can be found on-line or local hardware store, to reduce this, and a scriber or other fine pointed tool to pick out what actually manages to stick to the cutter.

https://www.dremel.com/au/en/product-categories/rotary-tools/accessories-attachments