I'll use a lot of things but I have to admit (rather lamely) that my personal preference runs to simple black linen Micarta. It takes a great polish, is totally stable, has a nice layered look when contoured, and doesn't crack, chip, or scratch easily. I'm no fan of the inconsistencies of natural materials, though they sure can be pretty. I feel G-10 is a little over-hyped; though a fine material, I think Micarta looks nicer, works easier, and the strength differences are negligible in 90% of its knife applications. Water buffalo horn is probably the prettiest natural material I've worked with, though dreadfully unstable, and Mesquite may be my favorite wood so far.
-Drew