Greco Aqualabs C6 stakes its claim as the superior GAC because of its very fine granular size [meaning higher reactive surface area) and greater porosity than other GACs available to hobbyists today.  These attributes reportly translates to C6 providing more efficient and greater chemical removal, with the caveat that users will have to use finer micron filter bags or lower flow rates through their media reactors (much like you would with ferric oxide media).  C6 removes hydrogen sulfide, chloramine, chlorine, and iron and is free of phosphates.

C6 is sold through ReadyAquatics.com (an Advanced Aquarist sponsor).  Prices start at $9.99 for 10 ounce and $14.99 for 20 ounce.

And I gotta admit, the graduated flask packaging is pretty darn cool!