I disagree on the Hikari. It is the best of the foods that use fish meal, but it still uses fish meal. Omega one uses no fish meal, therefore has more complete nutrition.
Like Hatchet said, a piece of pea once a week (I have heard that daphnia does the same thing, or you can do a day of fasting)
Puffers are more difficult to feed. They will not eat dried foods, and often won't eat frozen foods. They love to eat snails and shrimp, but will also take worms.
Endler's, as Hatchet said, can be fed by your favorite general-purpose fish food.
With all fish, but especially with the betta and dwarf, I would suggest giving treats of various sorts, such as freeze-dried bloodworms,
krill, bits of nori (the seaweed wrap used for sushi), etc...