Preheat grill to medium-high heat.
In a small bowl, mix together olive oil, chili powder, garlic powder, cumin, salt, and pepper. Rub the mixture onto both sides of the chicken breasts.
Grill the chicken breasts for 6-8 minutes per side, or until cooked through and juices run clear. Remove from grill and let rest for 5 minutes.
In a large bowl, combine cooked rice, black beans, corn, tomatoes, red onion, and cilantro. Toss to combine.
Slice the grilled chicken into strips and serve over the rice mixture. Top with shredded cheddar cheese and avocado slices.
Serve with sour cream and lime wedges on the side.