The advantage we have over violinists and pianists is that all the first great violists have been recorded; Tertis and Hindemith especially.
After that early era, viola really came into its own, especially since William Primrose, possibly the most famous violist of all time, who famously rejected violinists who played viola "on the side."
So I ask all of you: who do you think is the greatest violist of all time? I personally don't vote Primrose. I prefer Tertis, and among living violists I definitely vote Steven Dann.