Bring a large pot of salted water to a boil and cook the spaghetti according to package directions until al dente; drain and set aside.
In a large saucepan over medium heat, combine the chicken broth, cream cheese, hot sauce (start with 1/3 cup), 1/2 cup shredded cheddar, and Ranch dressing.
Stir frequently until the cream cheese and cheddar have fully melted and the sauce is smooth and warm, about 3–5 minutes.
Add the chopped cooked chicken to the sauce and stir to heat the chicken through and coat evenly; taste and adjust hot sauce as desired.
Toss the drained spaghetti with the buffalo chicken sauce until well combined.
Serve immediately, topped with sliced green onions and additional cheddar cheese if desired.
