A virtuoso is a performer with exceptional skills, both technical and expressive.
The term comes from the Italian virtù which means virtue, quality.
Initially it was used to indicate many types of qualities, both technical and intellectual. More and more the term has been used with reference only to the technical skills, sometime even with pejorative meaning, with reference to mere exhibition of technical ability.
More recently, famous viola virtuoso performers were
and William Primrose. In my opinion, a living true virtuoso (in the broadest sense of the word, as said above) is the viola player Michael Kugel.
Another example is Maxim Vengerov, a famous violin player who plays the viola too.
Now I'd recommend an aspiring viola virtuoso to hone his or her violistic skills by practicing with a collection of virtuoso pieces for viola and piano and also cultivate all other virtues, for example by reading about the life of great people and musicians, to see which ones they possessed and how they developed them. I find this always very fascinating, inspiring and encouraging.