In a medium bowl, whisk together olive oil, lemon juice, minced garlic, oregano, paprika, salt, pepper, and red pepper flakes. Add chicken to the marinade and coat well. Cover and let marinate in the refrigerator for at least 15 minutes.

Chop cucumbers, tomatoes, bell pepper, and red onion. Combine these with feta and parsley, seasoning with olive oil, lemon juice, salt, and pepper. Allow to sit at room temperature.

Heat a skillet over medium-high heat. Add marinated chicken breasts in a single layer, cooking for 3-5 minutes until golden brown. Flip and cook for an additional 3-4 minutes until cooked through.

Warm the pitas as preferred. Toast on gas flames, or heat in a skillet or oven until soft and pliable.

Assemble the tacos by filling warm pitas with sliced chicken, topped with salsa and tzatziki sauce. Serve immediately.