there is no 'best' and as far as functionality a lot of it is up to how well you cut and polish the slots.... tone is so subjective
make sure you get a good set of diamond coated fies for this type of work. You can buy special guitar fles but they are basically the same as good jewler's files and the jeewlery gear is generally cheaper for the same basic product.
most people like bone on vintage style fenders, they feel it sounds best and bone lasts a long time as long as the piece you get is void free... Tusq gets very close in sound, is always void free and it self-lubricates, but it will wear more quickly like other plastics. Depending on how heavy handed you are bone or metal may give you a lot more life. Bone lasts a long ass time, but its kinda hard to work with and the dust is very fine and kinda toxic. It just gets everywhere.
I do fine with plastics (though my best friend swears by bone and metal). Tusq is the best I've tried. I like the sound, its easy to work with and like I said above, it self lubricates. Vintage Gibson (original goldtop/burst les paul era) used nylon, which has a distinct sound, especially with a bridge humbucker. Its VERY easy to work on but as a result is the least durable material for a nut. You have to try it to get it. It has a sound. It binds and wears down easily though. Corian and a lot of other plastics just stink. They are harder to work with than Tusq and Nylon, they're not much more durable and they sound kinda blah.
Metals have a distinct sound especially on a strat. Brass adds a smidge of tele-ness (go the whole 9 yards with brass saddles and a brass sustain block if you want the fullest effect), They are harder to work on than Tusq but easier than bone (hard to believe, but its true, they're so dense that it feel easier to cut the grooves, though shaping them to fit your guitar can be a pain compared to bone and plastic).
All the tone differences, even the metals, are pretty subtle and no one but you will notice.
Cutting a new nut is a lot of work, even with a pre-shaped blank. Its rewarding but a pain in the balls.
