Preheat your grill or stovetop grill pan over medium-high heat.
In a small bowl, mix together garlic powder, onion powder, chili powder, cumin, paprika, salt, and pepper. Rub this spice mixture evenly over both sides of the chicken breasts.
Drizzle olive oil on the grill or grill pan. Place the chicken on the grill and cook for about 6-7 minutes on each side, or until the internal temperature reaches 165°F. Remove from heat and let rest for 5 minutes.
While the chicken is resting, combine the cooked rice, black beans, corn, diced tomatoes, cilantro, and lime juice in a large bowl. Mix well to combine.
Slice the grilled chicken and serve it over the rice mixture. Top with shredded cheddar cheese and a dollop of sour cream if desired.
Garnish with additional cilantro and lime wedges for extra flavor.