Place the cubed chicken into a plastic zipper bag or shallow container with a lid. In a small bowl, stir together the adobo sauce, lime juice, cumin, sugar, and salt until well combined. Pour the mixture over the chicken, making sure the chicken is thoroughly coated. Seal the chicken and refrigerate for at least 30 minutes, or up to two hours. When ready to make the tacos, heat the oil in a skillet over medium-high heat. When the pan is hot, add the chicken along with all the juices. Cook the chicken until browned on all sides and cooked through. Keep warm until ready to serve.
To make the crema, whisk together the sour cream, lime juice, garlic powder, salt, and milk in a small bowl.
To assemble the tacos: take each warm tortilla and fill it with the chipotle chicken. Top each taco with cilantro, onion, avocado slices, and crema. Serve with hot sauce or salsa on the side.