In the slow cooker, combine the chicken breasts, diced tomatoes, garlic, chili powder, salt and cumin. Cover with the lid and cook on low for 6 to 8 hours. Stir in beans and corn, and cook for 30 more minutes.
Use forks to shred the chicken directly in the slow cooker.
To serve, add rice or quinoa to a bowl. Top with shredded lettuce, the chicken and bean mixture, a sprinkle of cheese, avocado, pico de gallo, sliced green onions, cilantro and sour cream.