In a mixing bowl, whisk together the olive oil, chili powder, cumin, garlic powder, onion powder, paprika, lime juice, lime zest, salt, and pepper. Add the chicken breasts to the bowl and coat them well with the marinade. Cover and let it marinate in the refrigerator for at least 30 minutes (up to overnight for best flavor).
Heat a grill pan or skillet over medium-high heat. Once hot, add the marinated chicken breasts. Cook for about 6-7 minutes per side or until the chicken is cooked through and has nice grill marks. The internal temperature should reach 165°F (75°C).
Remove the chicken from the pan and let it rest for about 5 minutes. Once rested, slice the chicken into thin strips.
In the same pan, add the corn tortillas one at a time and heat them for about 30 seconds on each side, until they are warm and slightly charred.
Lay out the warmed tortillas. Add a generous amount of the sliced chicken to each tortilla. Top with shredded lettuce, diced tomatoes, avocado slices, and crumbled feta cheese if using.
Finish off with a sprinkle of fresh cilantro and an extra squeeze of lime juice, if desired. Serve the tacos immediately with lime wedges on the side.