Rinse the chicken breasts and trim any visible fat; pat dry with paper towels.
In a bowl or directly in a large resealable plastic bag, combine the olive oil, lemon zest and juice, chopped basil, honey (or agave), salt, and black pepper; mix to form the marinade.
Add the chicken to the bag with the marinade, remove as much air as possible, seal, and massage the chicken until evenly coated.
Refrigerate and marinate for at least 4 hours or preferably overnight.
When ready to cook, preheat the grill to high heat; if cooking from frozen, thaw the chicken in the refrigerator overnight first.
Place the chicken on the hot grill and cook for about 2 minutes until grill marks form and the chicken releases easily; if it sticks, wait 30 seconds more and try again.
Reduce heat to medium, flip the breasts, and continue cooking 8 to 10 minutes more, until juices run clear or the thickest part reaches 165°F (74°C).
Remove from the grill and let rest briefly before serving.