In a large bowl, combine the smoked paprika, garlic powder, onion powder, salt, black pepper, and cayenne pepper (if using). Mix well to create a spice blend.
Add the chicken thighs to the bowl and drizzle with olive oil and lemon juice. Toss the chicken in the spice mixture until evenly coated.
Cover the bowl with plastic wrap and marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.
Preheat your grill or a large skillet over medium-high heat.
Remove the chicken from the marinade and discard any excess marinade.
Grill or cook the chicken for 6-7 minutes on each side, or until the internal temperature reaches 165°F and the chicken is cooked through.
Once cooked, remove the chicken from the heat and let it rest for 5 minutes before slicing.
Garnish with chopped fresh parsley and serve warm.