What type of canister? For a slow-flow system (i.e., Phosban Reactor, Deltec Fluidized Filter, or Kent Phosphate Reactor), I'd recommend an iron-based media like Phosban, Rowaphos, or Magnavore PURA. I have only used Phosban, but I am going to give the Magnavore PURA Complete (combination carbon, resin, and phosphate media) a try.
If you're using a high-flow canister filter (i.e., Fluval, Magnum, etc.), your only real option is an aluminum-based media like Kent Phosphate Sponge or Seachem Phosguard. The down side of the aluminum-based media is that 1) they leach back phosphate after a few days and 2) can leach aluminum into the aquarium. The Seachem Phosguard says it will not leach phosphate and can be used continuously, but I don't know first hand.