Lightly grease the inside of your slow cooker with nonstick spray.
Place the chicken breasts or thighs into the slow cooker. Season them with salt, black pepper, garlic powder, onion powder, and smoked paprika if using.
Add sliced onions on top of the chicken if desired.
In a mixing bowl, whisk together Coca-Cola, BBQ sauce, Worcestershire sauce (if using), soy sauce, and red pepper flakes (if using).
Pour the sauce evenly over the chicken in the slow cooker.
Cover and cook on LOW for 6 to 7 hours or on HIGH for 3 to 4 hours, until the chicken is tender and easily shreds with a fork.
Using two forks, shred the chicken directly in the slow cooker and mix it thoroughly with the sauce.
Let the shredded chicken sit in the sauce for 10 to 15 minutes to absorb the flavors.
Serve over rice, on sandwich buns for pulled chicken sandwiches, or alongside mashed potatoes and roasted vegetables.
For a thicker sauce, mix 1 tablespoon of cornstarch with 2 tablespoons of cold water and stir it into the slow cooker during the last 30 minutes of cooking.
Optional: For crispy edges, transfer shredded chicken to a baking sheet and broil for 3-5 minutes before serving.