In a medium mixing bowl, whisk together the tequila, olive oil, lime juice, salt, pepper, onion powder, and agave nectar. Add the garlic and cilantro and stir to combine.
Pour the marinade into a gallon-sized zip-top food storage bag.
Add the chicken breasts to the marinade and allow to marinate in the fridge for at least 1 hour.
Preheat the grill to medium-high heat.
Use tongs to remove the chicken from the marinade, letting any excess liquid drip back into the bag. Place the chicken on the hot grill and cook for 5 minutes.
Flip the chicken over and cook for another 5 to 6 minutes or until the chicken is cooked through (internal temperature of 160 degrees F).
Remove from the grill to a plate, cover loosely with foil, and allow the chicken to rest for 5 minutes.
Remove the foil and serve warm with your favorite side dishes or slice the chicken for salads, burritos, or tacos.