I've used Kent Reef Carbon for the majority of my reef keeping, just finishing up a 3 gallon pail now. It is cheap(ish) and works well, so you can't go wrong with it. I bought some of the Brightwell Aquatics carbon to try next. I've used the BRS ROX in the past, and really like it as well. I don't really have a preference between ROX and Kent carbon performance-wise so I buy the Kent because it is cheaper. Whatever brand it is, I won't buy it unless it is pelleted because pellets wash cleaner and easier.
I use Aquarium Pharmaceuticals lignite carbon in my fish only tanks. I buy it by the box (4 or 5 gallons?). It's cheap crap.
...and not pelleted, although it rinses easily. I rinse using a stainless steel kitchen sieve. Have to change it more often...or use more than the better quality stuff.