Place the chicken breasts or thighs in the bottom of your slow cooker.
In a bowl, combine the cream of chicken soup, cream of mushroom soup (if using), ranch seasoning mix, chicken gravy mix, chicken broth, garlic powder, and onion powder. Whisk until smooth and pour the mixture over the chicken.
Cover the slow cooker and cook on low for 6–8 hours or high for 3–4 hours, until the chicken is tender and easily shredded.
Once the chicken is cooked, remove it from the slow cooker and shred or leave whole, as desired. Stir the sour cream into the gravy for extra richness, then return the chicken to the slow cooker. Let it warm through for another 10–15 minutes.
Serve the creamy chicken and gravy over mashed potatoes, rice, egg noodles, or with biscuits. Garnish with freshly chopped parsley for a touch of freshness.
