Legs are the best muscle part of the chicken. The skin, cartilage, spongy bone and marrow are also wonderful. But even if you only want the lean muscle, the leg and thigh are by far the best. I only eat chicken breast for health benefits (eating the whole animal is better, as sabertooth says), but I don't enjoy them. They're too bland. The legs and thighs and wings, I could eat all day.
I also don't eat frozen meats.