Boil ziti in salted water according to package instructions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add chopped onion and cook for 3–4 minutes until softened. Add garlic and cook for another minute. Stir in cream cheese, chicken broth, and ranch seasoning until smooth and creamy.
Preheat oven to 375°F (190°C). In a large mixing bowl, combine the cooked pasta, shredded chicken, bacon (reserving some for topping), and sauce. Mix until well coated. Stir in half of the cheddar and mozzarella.
Transfer the mixture to a greased 9x13-inch baking dish. Top with remaining cheeses and reserved bacon. Bake uncovered for 20–25 minutes, or until bubbly and golden on top.
Let it cool slightly before garnishing with fresh parsley. Serve warm.