Guns & Leather, The Outpost, Franklin Gun Shop... all seem to consistently have massive inventory. Hero Gear historically has had a massive inventory as well, although I have not been down there in at least a year and cannot accurately comment on how they are stocked now. Specialty Arms II has an good selection but honestly no more or less than what I've seen at other local medium-sized shops.
Sometimes size is not a good indicator of quality, though. While the aforementioned are all usually outstanding, I have likewise seen many desirable things and had good customer/staff interaction at some of the smaller/new shops. D&T Arms and The Armory, etc. all fall into that category easily and I'm sure their inventory has grown since I was last at either of them.
I guess the bottom line is that we are fortunate to have some really good shops around the middle TN area, many of whom are vendors here, some of whom have been and yet others who haven't. There really are few chances to "go wrong" within an hour of Nashville-proper.