I don't know what's the "best" (I've personally never tried fish food) but currently, I'm feeding four different varieties of Ken's food (http://kensfish.com/) per the numerous recommendations of many members and hobbyists throughout, and thus far, my fish appear to be loving it. The price is unbeatable in comparison to leading brands available in LFS's as well (Tetramin, Hikari, Wardley, Hagen, etc).
Ken's food is the best as far as I am concerned. Which is best for you depends on what fish you keep. Ken does have general foods, but you also can choose foods that are good for specifically your type of fish.
I was using four reputable brands (Tetramin included -- I still use what I have left interchangeably with Ken's) before I discovered and decided to try Ken's various foods. I can honestly say (and I'm pretty honest) that I'm glad I did. For the price you pay and the amount of food you get, Ken's the best. His service is quick and the quality of what he has to offer is :thumbsup:.
+1 for Kens. I feed my fish four different kinds of Kens food. It's the only place I can find veggie food for my herbivores that doesn't use fish meal as the main ingredient. And even with the shipping cost the prices are better than what you'll find at any LFS.
I'm using the guppy flakes right now and have the angelfish flake but haven't started using it yet. I just recently converted to Ken's foods (less than a month ago) so I don't want to comment on how it has affected growth, color, vitality, etc but my fish are definitely aggressively eating the flakes. In addition to the premium guppy flakes, I'm also feeding them the premium earthworm, premium brine shrimp, and the premium super tropical - all interchangeably and/or mixed to switch things up. I also bought the premium sinking four bit blend (sinking bits).