Place chicken breasts in slow cooker. Pour in water, then sprinkle chicken with taco seasoning and ranch seasoning. Cover and cook on high for 3-4 hours or on low for 6-7 hours.
About 20 minutes before serving, use a slotted spoon to transfer chicken to a bowl, shred with 2 forks, then return to slow cooker and stir.
When ready to serve, fill tortillas with shredded chicken, then drizzle with ranch dressing. Top with slaw, red onions, cilantro, and any additional desired toppings such as shredded cheese, avocado, or tomatoes. Serve immediately.
For a creamier chicken filling, stir 1/4 cup ranch dressing into slow cooker after shredding chicken. For a spicier chicken filling, add 1/2-1 teaspoon cayenne pepper to chicken along with taco and ranch seasonings.