I would say fiberglass or carbon fiber. It really depends on your setup honestly. A vented rotor setup isn't bad at all as long as there are some good thick pads involved. What vehicle??/
Is it the original Nitro RS4??? I have the HPI RS4 3SS and I am running the HPI Fiberglass disc brake and it has way more than enough stopping power for the car. If it is for on road the fiberglass disc should be enough. You may take a look at a carbon fiber disc and maybe a little something else. Is your car belt driven or shaft driven that would make a big difference as well. Still the fiber brake disc offered by HPI should honestly be enough.