You mean for spawning? Basically anything high protein. Too much mammal protein (meat or offal from mammals) is not good for them in the long run, but feeding a few times a week when conditioning should be okay.
Tiny bits of mince are good but don't just drop them in the tank, if they sink they will not be eaten and any meat in the tank rots the water very fast. I got mine taking food off the end of a toothpick very easily, I also feed termites and ant eggs in this way (DO NOT feed insects if you've been spraying insecticide, and don't feed flying insects if anyone in the house has been spraying anything like toilet deodoriser, air freshener, aftershave, perfume, burning incense etc.)
Most fish eat vegies but bettas are carnivores and not too keen on them... I stick with the meat. Tiny bits of cooked egg yolk, rolled into balls with the toothpick trick, are also okay (great! source of protein for conditioning) but only feed this right before a water change as any that is crumbled into the water will send it off.
They also love fish and shellfish flesh. What I do is freeze offcuts of fish or shellfish meat, then I use a cheap veggie peeler to shave off bits of the meat. You only need very light pressure, the veg peeler digs surprisingly deep into meat that's frozen solid. This can also be used to get small enough pieces of meat that hasn't been minced. Feeding fish or shellfish flesh is better than beef, pork or lamb as a source of protein (the mammal protein thing again) and egg is better yet.