This is a frequently asked question which unfortunately does not have a straightforward answer.  It's like a person asking, "Which is the best car to buy?" to which a person might ask back, "What are you going to use it for?",   If you want to carry passengers to school events you may want to consider a different vehicle than if you were looking for a car to commute to work everyday. Selecting  a variety of wood for carving is just as case sensitive as that. For this reason a person must be aware of the attributes of each variety in order to select the best for the particular subject they have in mind to carve.

You may consult a woodworkers textbook to aid in this determination but though these are excellent sources for specific attributes of many woods they really don't give a good indication to a woods "carvability" if I may coin a term.  Most woodworkers books consider the "workability" of the many varieties of wood, which is just to general to the purpose of use, and doe's not give a good indication of how it will perform under the duress of carving.