Combine garlic, onion, vinegar, ancho chili powder, cumin, oregano, 2 tablespoons of oil, salt, pepper, chipotle pepper, and adobo sauce in a blender and puree until smooth.
Put the chicken into a large resealable plastic bag and pour in the marinade. Seal the bag and massage it to coat the chicken evenly. Refrigerate for at least 2 hours or overnight for best flavor.
For grilling, warm the grill to medium-high and brush with 1 to 2 tablespoons oil. Grill chicken for 3 to 4 minutes per side until cooked through and the internal temperature hits 165°F. Work in batches if needed.
Let the grilled chicken rest briefly, then cut into chunks.
To cook on the stovetop, heat 2 tablespoons oil in a large skillet over medium-high heat. Cook chicken for 3 to 4 minutes on each side until fully cooked. Avoid crowding; cook in batches if necessary.
Allow the chicken to rest a few minutes before chopping.
For oven preparation, set the oven to 450°F. Arrange chicken in a baking dish and bake for 18 minutes.
Cover with foil and bake an additional 4 to 5 minutes.
Let the chicken settle before chopping roughly.