Preheat oven to 350°F (175°C). Cook manicotti shells according to package instructions until al dente. Drain and rinse with cold water to prevent sticking.
In a bowl, combine ricotta cheese, 1 cup mozzarella, Parmesan, garlic powder, salt, and pepper. Mix until smooth.
Fill each manicotti shell with the cheese mixture using a spoon or piping bag.
Spread 1 cup of marinara sauce in the bottom of a 9x13 inch baking dish. Arrange stuffed shells in a single layer over the sauce.
Pour remaining marinara sauce over the shells and sprinkle with remaining mozzarella cheese.
Cover with foil and bake for 25 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and golden.
Garnish with chopped fresh parsley before serving.
Notes
For extra flavor, add cooked spinach or ground beef to the ricotta mixture before stuffing the shells.