Hiya raider, welcome to MFK.
You will get a bit of controversy onthis question I'm sure.
A lot of people don't use feeders at all as all types of live food carry some risk.
One thing almost everyone agrees on is that the comet type goldfish sold as feeders are bad news, they tend to contain high levels of copper and other metals plus the occasional parasite and are in general not very nutritious as well as causing fatty deposits when used as a steady diet. If the goldfish came from someone who raises them for pond fish rather than feeders then IMO the occasional goldfish is okay but not as a steady diet.
The same goes for rosey reds.
Guppies are better but small.
Mollies are good but pricey.
Bait minnows and chubs are pretty good but it depends on where you are as to their availability and health.
I like using live sardines, anchovies, or smelt when I can get them but unless you are on the coast they can't be gotten.
Wild caught minnows, etc., are a parasite risk but good nutrition.
Small crayfish are good for fish that will take them as are glass shrimp, sand shrimp, and mysids.
Live crickets are very good as are farm raised earthworms.
If you use live fish as feeders there are a couple precautions that it is good to take,
Get yourself a large tub and a few large box filters so you can quarentine them in a tank salted like you were treating for ich, 2 teaspoons of salt per gallon, or one level cup per 24-30 gallons of water, and keep them in it for 10-14 days before use. Feed them on good quality foods.
If you have to use them quickly then give them a bath first in a salt water solution of 2-3 tablespoons of salt per gallon for 2-3 minutes and then a quick rinse in freshwater before feeding them to your fish. this will remove most external parasites.
I am a fan of live foods and have had good results with it but I am also a believer in varied diets for the fish. A mix of worms, shrimp, gut loaded crickets, and feeder fish with some pellets will keep your fish in good color and condition where often a steady diet of just one item can lead to dietary deficiencies.
As I said earlier, there are people who have had different experiences than I have and this is just my opinion but it has worked for me.