Place a large skillet over medium heat and cook bacon until crispy, turning as needed, about 10 minutes. Transfer to a paper towel-lined plate to drain. Once cooled enough to handle, finely chop the bacon.
While the bacon cooks, cook the spaghetti in a large pot of salted boiling water for 8 minutes. The spaghetti will be slightly undercooked - it will finish in the oven. Drain the pasta and set aside.
In a large mixing bowl, whisk together the soup, cream cheese, and sour cream. Pour in the milk and ranch seasoning and whisk to combine.
Add the chicken, ½ of the shredded cheese, and ¾ of the chopped bacon. Stir into the sauce.
Spray the inside of a 9x13 baking dish with nonstick spray. Add the spaghetti to the baking dish. Pour the chicken mixture over the pasta and carefully stir to combine. (If you have a giant mixing bowl, you can mix everything together then pour it into the baking dish.)
Bake for 20 minutes. Remove the dish from the oven and sprinkle the pasta with the remaining shredded cheese and bacon. Bake for another 10 minutes.
Remove the casserole from the oven and sprinkle with chopped parsley. Cut into 6 to 8 portions and serve warm.
Notes
This casserole can easily serve up to 8 people if you're having a salad or other side dish along with it.