The best nappy bag is the one that suits all your needs.

I use a krumpler courier bag, can go over shoulder, or has an extra strap to go on back.
is water proof, and has little pockets inside.

if you buy expensive you will want to use it, my sister just sold her oi oi nappy bag for $5 at a garage sale, it was in really good condition! at a second hand baby shop would have easily gotten $50