Bach's 'Six Suites' are the Holy Grail of Unaccompanied Pieces for Cello. They have been transcribed for multiple instruments including violin and viola.
Cellists often sneer at violists who perform these works. However, historical evidence suggests that these violists might not be as far off as cellists might think. What if the works were really written for a type of viola and they've been transcribed for cello??? Oh, the horrors!