Preheat the oven to 425 degrees F. In a large skillet, heat the olive oil over high heat. When the oil shimmers, add the ground chicken and onion. Cook, breaking up the meat as it cooks, until the chicken is browned, about 5 minutes. Add the chipotle chili powder, paprika, cumin, garlic powder, red pepper flakes, oregano, and salt. Add 3/4 cup water. Reduce the heat to medium and simmer until the sauce has thickened slightly around the chicken, about 10 minutes. Remove from the heat and stir in the cilantro. Line the taco shells up on a sheet pan or in a 9x13 inch baking dish. Transfer to the oven and bake 5 minutes. Evenly divide the meat among each taco shell and top with cheese. Bake 10 minutes, until the cheese has melted. Meanwhile, make the ranch. Combine all ingredients in a glass jar and shake or stir until creamy. Serve the tacos topped with ranch, avocado, red onion, and any other desired toppings.