In terms of overall population, the Proof 1900 Quarter Eagles are the most plentiful of the Liberty type. However, that does not mean they are common by any means - only that this is the least rare date in the series.
In general, the quality of Proofs in this year is excellent. Quite a few high grade examples exist, even up to PR68+DCAM, and a high percentage of 1900 Quarter Eagles are Cameo or Deep Cameo. For type purposes, this is the date of choice.