You all seem to have forgotten one very important person.I belive this makes him the highest ranking aikidoka alive today.
Minoru Mochizuki who received the Menjya Kaiden (Certificate of mastery from Ueshiba) and was recognised in 1980 by Ueshiba O'Sensei's son, Kisshomaru and the International Martial Arts Federation when he was nambed Judan, (tenth-degree black belt), and meijin (worlds highest technical representitive),in Aikido.
Read up on Mochizuki you will be impressed!
Not to quibble (because if anyone deserves a 10th dan it's probably Mochizuki), but his rank was not processed through the Aikikai, which means that the only *Aikikai* 10th dan promotion (officially) is still Koichi Tohei. The International Martial Arts Federation wanted to promote him, but he wouldn't take it without Doshu's (Kisshomaru) approval (which he got) - but the rank was never officially processed or certified by the Aikikai, it was granted by the International Martial Arts Federation.