This is a broad question. Is carving glass hot or cold?
Carving hot, there is not a best tool. It is more of what tool works well for the artist. Some great materials are graphite and even a knife. The tool needs to be able to withstand a lot of heat
For carving cold there are also many tools, and rather than the best, what is the best tool for the project? Some use sandblasting and just a graphic. Some use a diamond bit in a Dremel and some use a diamond saw.