I'd have to include my own instructor, Jon Eley Sensei. Even though he is a 6th dan and has been doing Aikido since it started in Chicago (40+ years), most people in Chicago and the Ki Society don't know of him. However, when he teaches, I always see and learn something new. This has been going on for 14+ years.
Another interesting teacher is Mikoto Masahilo Nakazono (1918 - 1994). Sensei Nakazono studied Aikido directly under O Sensei Ueshiba, the founder of Aikido, for many years in Japan. He left behind his thoughts on Kototama. Although he is not as well known as Stevens, Gleason or Barrish, he is one of the few direct links to understanding O Sensei's spirituality.
One of his former students is selling reprints of his essays at