Spray the inside of a slow cooker with cooking spray.
In the slow cooker, combine the zesty Italian dressing, minced garlic, chili powder, ground cumin, dry ranch dressing mix, and water; stir until evenly mixed.
Add the chicken breasts to the slow cooker and turn to coat them in the seasoned liquid.
Cover and cook on LOW for 4–6 hours or on HIGH for 3–4 hours, until the chicken is fully cooked and shreds easily with a fork.
Remove the chicken to a cutting board or bowl and shred it with two forks or tongs.
Return the shredded chicken to the slow cooker and stir to coat with the cooking juices and seasonings.
Serve topped with chopped Roma tomatoes, olives, minced cilantro, fresh lettuce, and cilantro ranch dressing as desired.
