Hands down I always go for row 13 on an A320. In my experience over the years, exit rows tend to have less people in them on Frontier because they are the last to get assigned unless someone selects them. Personally, I value the space beside me more than legroom. I even go for row 12 (the first exit row) sometimes due to the fact that I am less likely to have someone next to me. Be aware that row 12 has extra legroom, but the seat back is more upright due to the exit behind and more uncomfortable than other rows. I avoid the first 3 rows unless the plane isn't very full.