Thankfully Sir Reginald isn't a picky eater, so I'm still using the thing of Topfin Betta food I bought when I bought him. The only thing I don't like about it is some of the pellets seem large.
I've heard great things about the Hakari Bio Gold, so thats my next food purchase. Once the Topfin gets about half empty. The bio gold also has the most reviews on Amazon, and the highest review average at 4.5 stars. New life specturm comes in second on amazon with an average of 4 stars. I'm curious about Seachem's Betta Banquet, but I have fond no reviews good or bad.