Record City is good but I haven't been in a while. From what I remember, Zia (at least on Rainbow) has a better Hip-Hop section than RC. Wax Trax is famous/infamous but I don't think it's the best.
I haven't been to Moondog in a while (they've since moved) but I liked them when I went and owner would put on records you were interested in to sound test. 11th Street is what VMP claimed was the best record store in NV. Haven't been through there in a bit. Remember it being a nice store, if pricey. Take VMPs rec as you will.