What is the best abrasive to use?

The answer depends on how much pressure is being used and even then, the recommendations need to be considered only as a “general guideline”:
Sanded Material Best Better Good
Carbon Steel & Cast Iron
Moderate to Heavy Pressure Ceramic Plus Zirc Plus / Zirc AO
Light Pressure Zirc PC AO
Stainless Steel & Exotic Metal
Moderate to Heavy Pressure Ceramic Plus Zirc Plus AO
Light Pressure Zirc PC AO Plus AO
Aluminum**
Moderate to Heavy Pressure Ceramic Plus Zirc Plus AO
Light Pressure AO Plus AO SC
Non-Ferrous (Titanium, Copper, Brass)
Moderate to Heavy Pressure Ceramic Plus Zirc Plus / Zirc AO
Light Pressure AO Plus AO SC
Wood
Pressure does not affect performance Ceramic OC AO Open Coat AO
** Due to the inherent clogging involved when sanding most Aluminum, we recommend either using products with Grinding Aids or using a Grease Stick to lubricate the abrasive to help prevent loading.
Legend:
Ceramic Plus: Ceramic Alumina with a grinding aid on a heavy Y-wt. polyester backing
Ceramic OC: Ceramic Alumina Open Coat on a heavy duty backing
Zirc Plus: Zirconia Alumina with a grind aid on a heavy Y-wt. polyester backing
Zirc: Zirconia Alumina on a heavy Y-wt. polyester backing
Zirc PC: Zirconia Alumina on a X-wt poly/cotton backing
AO Plus: Aluminum Oxide with a grinding aid (available on Y-wt. or J-Flex backing)
AO: Aluminum Oxide (available on Y, X or J-wt. backing)
SC: Silicon Carbide on a heavy Y-wt. polyester backing