Hmm, the best food is a variety of types. I know bettas can be fussy, but proper nutrition is better for them (eat your green beans honey!)
So, I feed HBH Betta Bites, they used to eat Hakari Bio-Gold but that food tends to swell their stomacs badly so they can get swim bladder issues.
I also give them a weekly treat of Freeze-dried bloodworms, but that should NOT be the primary food source.
Believe it or not, Wardey Tropical Flake Food is a great food source, if your betta will eat it. It has a good variety of meat sources.
Live and frozen foods (frozen moquito larvie, water fleas, bloodworms) are terrific (but expensive), however, that should not be the only source because they can contain parasites that can infect your bettas.
Variety will help them digest better and not miss-out on vitamins and protein. Read the fish-food ingreadients, you want fish meal and 'meat' based products first on the list, not wheat/grains for bettas.
Also, you should skip a day of feeding once a week. I don't feed my bettas on my weekly water change day. This gives them time to empty their stomacs and relieve constapation.
Bettas will and do over-eat. They don't stop eating when they are full, so their stomacs get really bloated. Its up to you to dicipline yourself not to over-feed them. If you want to have your betta live a long time, reduce the food amount. More food will speed up their metabolism and shorten their lifespan. A hungry betta is a normal betta, they are always hungry.