Place the chicken breasts in the bottom of your slow cooker.
In a separate bowl, mix the cream of chicken soup, diced tomatoes with green chilies, taco seasoning, and water until combined.
Pour the soup mixture over the chicken in the slow cooker.
Cover and cook on low for 6 to 8 hours, or on high for 3 to 4 hours, until the chicken is fully cooked.
Remove the chicken breasts from the slow cooker and shred them using two forks.
Return the shredded chicken to the slow cooker and stir it into the sauce to coat completely. Let it heat through for 10 minutes.
Warm the corn tortillas according to package directions (you can warm them directly over a gas burner for a few seconds or wrap them in a damp paper towel and microwave briefly).
Spoon the shredded chicken mixture into the warm tortillas and add your desired toppings.
Notes
For juicier chicken, use chicken thighs instead of breasts.
If you do not have cream of chicken soup, substitute with 1 cup of chicken broth and 1 tablespoon of cornstarch mixed with 2 tablespoons of water added at the end to thicken the sauce.
This shredded chicken filling can be made ahead and stored in the refrigerator for up to 4 days.