I prefer a floating pellet. It's easier to observe the fish feeding. I have used Laguna's feed, both the small and medium pellets depending upon the size of the fish:

http://www.amazon.com/Laguna-Goldfish-Floating-Medium-Pellet/dp/B0035H9IG2/ref=sr_1_1?s=pet-supplies&ie=UTF8&qid=1404880163&sr=1-1&keywords=laguna+koi+food