Applejack. She's the only character that approaches the three-dimensional status, having a near complete family, a job, a realistic personality, a "good" accent, and she's so great they had to turn her into a background character because people want to see her more.
After AJ probably Pinkie, since she does have a family and a job, but she's not far from being one-dimensional thanks to that silly toon personality of hers.
I also consider Fluttershy, but she doesn't exactly have a job nor a family yet. She does seem realistic and relatable to most bronies who are shy to meet others.