Preheat oven to 350°F (175°C).
In a large bowl, combine sour cream, cream of chicken soup, mayonnaise, onion, celery, bell pepper, garlic powder, salt, and black pepper.
Stir in the cooked chicken and rice until everything is well coated with the creamy mixture.
Transfer mixture into a greased 9x13 inch baking dish.
Sprinkle shredded cheddar cheese evenly over the top.
If desired, mix breadcrumbs with paprika and sprinkle over cheese for a crunchy topping.
Bake uncovered for 30–35 minutes, until bubbly and golden on top.
Remove from oven, let rest 5 minutes, and serve warm.