ground beef. Don't go with the lean stuff, otherwise they turn out dry. (Don't worry, a lot of the grease will cook out of them.)
I prefer making it with bread crumbs, but if you don't have bread crumbs on hand, this will work well:
soak 4 slices of bread in some water for a minute or two. Mix in 2 eggs, add 1 lb ground beef, about 1/4 cup grated parmesan cheese, 2 Tbsp parsley, about a tsp of salt, 1/4 tsp oregano, and a dash of pepper. Form into balls, brown slowly in a bit of oil. Add to spaghetti mixture & let cook for at least a half hour. I personally think it tastes best made a day in advance, then allowed to heat up and simmer in a crockpot for a couple hours the day you're going to use it.
p.s. Someone mentioned Italians. A true Italian connoisseur would consider it blasphemy to use canned or jarred spaghetti sauce. Start with the freshest Roma tomatoes you can get. Not the picked green, artificially ripened crap sold at most grocery stores...