In a bowl, mix together olive oil, chili powder, cumin, smoked paprika, garlic powder, onion powder, salt, and pepper to create a marinade.
Add the chicken thighs to the marinade, ensuring they are well-coated. Allow the chicken to marinate for at least 30 minutes (or up to overnight in the refrigerator for deeper flavor).
Preheat a grill or skillet over medium-high heat. Remove the chicken from the marinade and place it on the grill or skillet.
Cook the chicken for about 6-7 minutes on each side, or until it reaches an internal temperature of 165°F (75°C) and is no longer pink in the center.
Remove the chicken from the heat and let it rest for 5 minutes before slicing it into bite-sized pieces.
In a separate pan, warm the corn tortillas for about 30 seconds on each side until pliable.
Assemble the tacos by placing a portion of the sliced chicken on each tortilla. Top with chopped cilantro, diced red onion, avocado slices, and jalapeños if using.
Squeeze fresh lime juice over the tacos for added flavor just before serving.