On Duty it's the issued nylon crap.
Off Duty I prefer leather. In fact I prefer custom leather that I have to wait for. Matt Del Fatti and Milt Sparks make great holsters, and they really aren't too much more exspensive than the "off the shelf" stuff, but are heaps better in comfort and concealability.
One other thing; always use a good gunbelt with your holster. The belt is the foundation, much like the foundation of a house. If the foundation is no good it doesn't matter what they house on top of it is like.
Biker