In a mixing bowl, whisk together the olive oil, lime juice, chili powder, cumin, garlic powder, salt, and black pepper to create a marinade.
Place the chicken breasts in the marinade, ensuring they are well-coated. Let them marinate for at least 15 minutes (or up to 2 hours for deeper flavor).
Preheat the grill to medium-high heat.
Once the grill is hot, place the marinated chicken breasts on it. Grill for about 6-7 minutes on each side or until the internal temperature reaches 165°F (75°C) and the chicken is cooked through.
Remove the chicken from the grill and let it rest for a few minutes before slicing it into strips.
While the chicken is resting, warm the tortillas on the grill for about 30 seconds on each side until they are soft and pliable.
To assemble the tacos, place some sliced grilled chicken on each tortilla and top with shredded lettuce, diced tomatoes, shredded cheese, cilantro, and sliced jalapeños if desired.
Serve with sour cream or guacamole on the side.