It really depends on what fish/inverts you have, how busy you are, and how much money you have to spend.
I personally feed a selection of frozen food every second day, with a small amount of pelleted food on alternate days.
This is because my fish are mainly large carnivores (well, not that large yet, but growing). Carnivores are generally better fed to satiation then not fed for a while. This makes them produce less waste, and also helps prevent some illness.
I also add nori on occasion to supplement the algae in my tank, this is for my rabbitfish - which is mainly a grazer, although even the carnivores will have the occasional nibble.
Some fish require feeding multiple times a day, this may cause problems if you work, and so automation is often useful here.
Some invertebrates and fish do best with a constant supply of food. This may involve drip feeding/regular dosing or maintaining a live supply of food in the aquarium.
As to what to feed - I believe that frozen or fresh food is much better than flake/pellet food, which I do use, but only really as a small percentage.
Currently I use a fairly wide range of frozen food, including - brine shrimp, mysis, krill, chopped cockle/mussel, whole cockle/mussel, squid (big favourite with the fish), fishmeat, spirulina, cockle in shell (important for triggers/puffers etc. to keep their beaks trimmed) and others.
Once my fish get a bit bigger, standard packs of frozen food will become un-economical and I intend to use more frozen prawns etc. which can be bought cheaply in decent quantity.
I am also considering setting up some sort of live food breeding system - perhaps mysis.
I find my fish much prefer pelleted food to flake (exactly the opposite to my tropical freshwater fish, who really don't like pellets).
Note - when using frozen food please thaw it thoroughly first! This may seem very obvious, but a lot of people don't, and you can potentially kill fish this way.
I have sometimes been guilty of being in too much of a hurry - only to see my porcupine puffer consume a large cube of food, only to regurgitate it a few seconds later.
Many people also add various supplements to their food before feeding - for example garlic which can help immune systems and prevent disease, or vitamins.
