I think it depends on the fish...
My mbuna get their food based on ingredients and how the fish look over time. As anyone who keeps mbuna knows...they are beggers. They'll eat anything. I could feed them the "friskies" of fish food and they'd want more. I've noticed that a lot of food sold as "spirulina" or "veggie" still has fish meal as the 1st ingredient. I'm not sure where in the list it should be for mbuna, but I try to mix it with other green-type flakes that have it further down the list.
On the other hand, I have the new WC pair of calvus that I got at the ECC auction. They've been hiding (not unexpected) but I found that they go nuts for HBH's Super Soft Krill, which I recently got just to try. Soooo, since I'm pretty much ok with HBH as a decent brand...they get the krill because of how they are behaving. They've even started coming out of hiding before I feed them. Of course...if I offer anything other than the krill right now, I get a look of deep disappointment from them (yeah, yeah, i'm anthropomorphising, oh well).
I also think diversity is important. I don't feed my fish the same thing all the time. They may get flakes a few days in a row, then they'll get some NLS.
And, of course, I am VERY guilty of overfeeding them, so i'm working on some personal behavior modification in that area, too. Anton's talk was an excellent reminder.