While there's no way to determine the best sauce for meatballs, they work well in tomato sauce. Savory sauces such as those used with Swedish meatballs and those that add sweetness through the use of ketchup and brown sugar are also popular meatball sauces.
To make an easy tomato sauce, cook one finely chopped onion in olive oil until it's translucent. Add 2 pounds of pureed tomatoes and bring the sauce to a boil, adding salt and pepper to taste. Tomato sauce can be customized by adding a variety of Italian fresh or dried herbs and spices, such as basil, parsley and oregano.
To make Swedish meatballs, make a roux by melting 1/4 cup of unsalted butter and adding 1/3 cup of flour. Cook the roux for a few minutes until the flour and butter are well-blended, then whisk in 4 cups of beef broth. Whisk constantly while cooking until the broth is slightly thickened. Stir in 3/4 cup of sour cream and salt and pepper to taste.
Another popular sauce for meatballs is a ketchup-based sauce made on the stovetop. In a large skillet, combine 1/2 cup ketchup, 1/2 cup dark brown sugar, 2 tablespoons water, 1 1/2 tablespoons cider vinegar and 2 teaspoons Worcestershire sauce. Add 1 tablespoon finely grated shallot, 1 small minced clove of garlic, 1/8 teaspoon ground black pepper and 1/2 teaspoon salt. Cook for 10 minutes until the sauce is slightly thickened. Add the cooked meatballs to the skillet, and toss to coat them lightly with the sauce.