The 1901-S Barber quarter is, in my opinion, the rarest regular issue (non-variety) silver coin of the 20th century. The original mintage of 72,664 is very low for the era. For example, the mintage of the 1901 was 8,892,000, the 1901-O was 1,612,000, and the 1902-S was 1,524,612. And the 1901-S was issued at a time when collecting branch mints was not the general practice. Most collectors at that time just sought one of each date, regardless of mint. Collecting by date and mint mark didn't start to get popular until about ten years later.
The majority of 1901-S quarter surviving today are very low grade with the typical specimen being an AG3 or GD4. Higher grade specimens are rare and mint state specimens are very rare. There are a small handful of Gem MS65 or better specimens, I would estimate the number to be between 10 and 20.