Cooking meatballs can be fun, but figuring out the ideal meat-to-fat ratio can be tricky, depending on whether you're making meatballs for sauce or broth.
Ground pork is pretty fatty; you can find it in the grocery store with meat-to-fat ratios ranging from 80-to-20 to as fatty as 60-to-40 and even 50-to-50.
If you're using fairly fatty pork, choose leaner grinds of the other meats like 90-to-10 ground beef and 80-to-20 veal, as the pork fat will do all the heavy lifting.
If you're making meatballs for broth, use leaner meats because they don't need to cook as long, and you want to avoid extra fat accumulating in your broth.
For a broth with a beef base, choose a blend that's very lean, like a 90-to-10 meat-to-fat ratio, and try not to choose any veal with more fat than a 70-to-30 blend.
Ground chicken and turkey are naturally very low in fat but are typically pre-packaged. Although you can't choose the grind, they are great for making meatballs for broth and soup.
You can always look for a "meatloaf blend," which usually has around an 80-to-20 blend of fat-to-meat with a mix of beef and pork, which will work fine for most meatballs.