Bring a large pot of salted water to a rolling boil. Cook penne 1 minute shy of al dente, reserve 1/2 cup pasta water, then drain without rinsing.
Season chicken with salt, pepper, garlic powder, and onion powder. Sear in olive oil over medium-high heat, 6-7 minutes per side, until 165°F. Rest 5 minutes, then chop or shred.
Sauté onion and celery in olive oil over medium heat for 3-4 minutes until softened. Add garlic and stir 30 seconds until fragrant; season with salt and pepper.
Pour in chicken broth and scrape up browned bits. Add cubed cream cheese and stir over medium heat until fully melted and smooth, 2-3 minutes.
Stir in buffalo sauce, ranch dressing, garlic powder, and onion powder. Taste and adjust salt, pepper, or buffalo sauce.
Reduce heat to medium-low. Add cheddar and mozzarella in small additions, stirring until smooth and glossy. Loosen with reserved pasta water if too thick.
Stir in the cooked chicken and warm through for 1-2 minutes. Add drained pasta and toss until every piece is fully coated.
Divide into bowls and top with green onions, crumbled blue cheese, and a drizzle of ranch or blue cheese dressing. Serve immediately.