Preheat oven to 350°F (175°C).
Cook manicotti shells according to package instructions. Drain and set aside.
In a large bowl, mix ricotta, mozzarella, Parmesan, egg, spinach, garlic powder, basil, salt, and pepper.
Stuff each manicotti shell with the cheese mixture.
Spread 1 cup of marinara sauce on the bottom of a baking dish.
Arrange stuffed shells in the dish and cover with remaining sauce.
Cover with foil and bake for 30 minutes.
Remove foil and bake for an additional 10 minutes.