All of them: for different reasons. Dragons are very intelligent, and their purposes for immortality are extreme. The northern ones, for instance, are the guardians of their icy land. The western dragons, the guardians of healing, and so on.
Elves, however, are quite shy: if you stick to how they are normally portrayed. But their forest lore is great and they know how to live in the forest and mountains like no other.
Trolls - well, despite their ugliness and stupidity, their saliva is a fantastic healing property for deep wounds and sleep disturbances. :)