KAMI : The highest ranking grade in Aikikai is 9th Dan (the one and only 10th Dan was Koichi Tohei Sensei when he was still a member and Head Instructor of Aikikai Hombu Dojo).
Who are/were the highest ranking aikidoka, and what was their rank!? (I presume O sensei was the highest rank - 10/12 dan?)
That's basically correct, with some qualifications (which is why it's such a messy issue).
1) *Currently* the highest ranking people in the Aikikai are 9th dan, I don't think that 10th dan has ever been officially ruled out (which is what the post above seems to imply).
2) Koichi Tohei is the only 10th dan to be officially processed through the Aikikai promotion system. Morihei Ueshiba promoted a couple of other people to 10th dan himself - whether those are "official" 10th dans or not depends on who you talk to. At least one of those people has "Aikido 10th dan" printed on his business cards. The Aikikai policy on this seems to be "don't confirm/don't deny", meaning that they won't officially recognize that promotion, but they won't officially deny that it exists either. This is a pretty Japanese solution to an awkward situation - ignore it and pretend that it doesn't exist. In the western legal system, of course, there would be an argument of "silence implies consent", so again, it depends who you talk to.
3) There were a couple of people that Morihei Ueshiba gave "menkyo kaiden" certificates. This is outside the kyu/dan ranking system, so I suppose that it depends on your point of view, but "menkyo kaiden" ("certificate of complete transmission") is usually the highest level certificate presented in an art, and implies that the person giving it has taught the person receiving it everything in the art, so some people may consider that equal to (or greater than) a 10th dan.
So far as I know, Morihei Ueshiba never held *any* dan rank himself, and he seemed to do OK :-). Off hand, the only verifiable certification that I recall is his Daito-ryu "kyoju dairi" certificate, which is an assistant instructor's license.