bulk reef supply hi capacity gfo or bulk ganular. Everything that's prepackaged is 10x the price, and I've definitely had sketchy results with some of the store bought media. I've been using hi-cap for over a year now, and just recently changed to the granular, just to save a few bucks. I swap it out monthly.
...edit... I should edit this, I had good result with ROWAphos, it was just more money. The cannisters are pretty cheap, nothing wrong with daisy chaining the carbon reactor to a gfo one. That's how I do it, one month carbon is first in line, second month the gfo is first, etc