Well, it would have to be wood, naturally
I've had over 200 beautiful pieces of wood come and go over the last 3 years or so, and each had it's own particular charms. I remember one piece of Mopane, which is usually a fairly uniform looking medium brown wood. It wasn't until I applied a finish to the turned tube that I discovered some really fascinating iridescent grain lurking below the surface. I had a brief experience with Pink Ivory which was gorgeous and very exotic but caused me no end of heartache with cracking. It just may have been the batch I had, and I'm tempted to give it another try.
Tulipwood has always excited me. I once played a fabulous Von Huene recorder in Tulip with ivory fittings (pre-ban), and when I bought my first Thin Weasel many years ago, one of the things that led me to call Glenn was to ask him if he could make me one in Tulipwood.
I think the most exotic stuff to pass through the shop has been Snakewood. Simply amazing stuff. Also finicky-- all 4 whistles I made from it developed cracks a couple of months later! Fortunately, repairs were easy and the repaired cracks actually look like part of the grain and are undetectable unless you know where to look with a magnifier. Actually, not all of that Snakewood passed THROUGH my shop-- I kept one of those beauties.
Note that I'm not touting my own whistles specifically, just some of the beautiful materials I've been lucky enough to work with.
I got it at Andy's Front Hall (now gone sadly) near Albany in Busman country as they were packing it up for a show; it never made it to the show. And, yes, I paid for it. 
) - gleaming silver color, file work around the tuning slide, and a beautiful marbled Albanite plug.
