I use Aqueon betta pellets, and I highly recommend them! It has all natural ingredients,plus added vitamins and minerals. My bettas love them! I also feed mine some freeze dried bloodworms, and I use the brand topfin. My bettas eat those right up! :) :)
I give mine a variety of pellets, (NLS) frozen/FD bloodworms, tropical flakes, frozen daphnia, shrimp pellets, and sometimes algae wafers. Okay, I don't really give them shrimp pellets and wafers, they usually steal some of the cories' food. Lol.
Just an alert to the other people who are looking at this thread. The pellets by Hikari, Aqueon, Wardley, Top Fin, and Tetra are horrible quality. It's not about "natural ingredients" that they say on the front. Look for protein based ingredients, (i.e. whole halibut meal) instead of cheap protein boosters. (i.e. wheat gluten) You can see that they hardly have any of those. Look at the first 3 ingredients, see what I mean. Hikari used to be the best but they switched it over to the bad side. Some of the BEST ONES are New Life Spectrum, Golden Pearl, Omega One, and Attison's Betta Pro. They discontinued the Pro however, I *think* the Betta Food is okayish..
Based on my knowledge of cat food and digestion I can say that even a by-product meat is better than a meat substitution (IE, plants). That is, even a low quality meat will be digested better by a cat than any plant ever will be because they are carnivores, and obligate carnivores at that. I don't know if bettas are obligate carnivores, but I imagine they would be (though I would love a second opinion on this and facts either way!). Just like I feed my cat a meal that is about 80% meat, 10% bone, and 10% organs, I imagine the ratios are similar for an ideal betta diet. Except instead of bone you would have exoskeleton. And to be honest I wouldn't know what the "meat to exoskeleton" ratio would be either, so those percentages would probably be off.
Of course if they are not obligate carnivores then I image they would eat something more similar to what my dogs eat. Which is actually the same thing that my cat eats. But the important difference is that they could eat and actually digest some plant matter (which again, cats are completely unable to do). Based on a betta's inability to digest say, a pea leads me to further believe they are probably obligate carnivores, but like cats, might like to chew on grass sometimes for whatever reason.