Asking an artist to pick their favorite medium or their favorite work of art is akin to asking a parent to choose which of their children is their favorite. Ok, I don't have any children, so I may be reaching a little, but still it is very difficult to choose a favorite. But, I will at least list a few of my favorites, in no particular order.
Amboyna burl is a spectacular wood to work. It has rich red colors with various highlights, browns and creams. Typically, the grain is spectacular, with tight swirls, wonderful eyes and just all sorts of unusual patterns. It almost always yields an amazing looking pen. It is also a predictable wood to work with, usually dense and relatively easy to turn, that is, for a burl. Clearly, a standout in the world or wood.
Redwood Lace Burl is a tight-grained wood with exceptional character. Often the wood has a lace like grain pattern, which makes the wood look like it has waves in the grain. The color is typically a red (to orange-red), with deeper brown colors running through the wood. Since it is a redwood, the wood has a characteristic redwood smell when turning the wood. This is a pleasant way to make the shop smell nice. It is a softer wood, easy to turn, but often needs some extra work to stabilize the wood.