A lot of people use Attisons Betta Pro, or Hikari. They're small pellets. I feed mine Tetra Prima Mini granules. Since none of my LFS stock either Attisons or Hikari ( plus he loves his current food so I'm not about to change it )
Live foods are also needed. Bloodworm, Daphnia and Brine shrimp are all good live foods, or you can use frozen and defrost portions as you require them.
I also like to feed small young earthworms washed and cut up into manegable little bits. A slightly unpleasant job, but well worth it for the reaction . My bettas go BESERK for earthworm. You must only use worms from a compost bin though, never straight from the ground ( as they may have passed though soil that has been treated wih something, and this can be passed to the fish )
Don't feed live or meaty foods every day though. You don't want to bloat the fish. feed 2-3 times per week, and one day a week, feed either nothing at all or a little bit of a cooked, de-shelled crushed up pea. This cleans out the bettas system and guards against bloating. A bit like a detox day .