Least used First Letter...
 

I was looking through the page on Wikipedia that has Greek Letter Organizations sorted by Greek Letters so I was looking for the least common. Excluding a Philippine founded GLOs, there is a five way tie at three groups a piece:
Epsilon, Eta, Xi, Omicron and Upsilon. All except Xi are vowels...


(507 Greek Letter Organizations listed)
https://en.wikipedia.org/wiki/Catego...zation_letters