I don't think a persons level of Aikido is judged by his ability to teach it.
However, the potential for people to be more technically proficient than Ueshiba M. at any particular age (ie. person at 30 when Ueshiba was 30) certainly exists.
There are a number of reasons for this including; age started, raw talent, time spent under direct tuition.
I've heard one person described as the "best Aikidoist around - possibly ever" but I am forced to take the statement with a grain of salt. Not because the person is not seriously awsome but that no one knew him or Ueshiba M. when both were in their prime. I am sure others know people that may also fit the above bill although I suspect each and every one of them would deny the fact. And really, although Ignacio's response was a bit reactionary, who is going to judge?
By the way I also don't think O'sensei needs to imply technical dominance or even a superior teaching ability. We don't use the O sensei term, its usually just Ueshiba sensei, but really its just a cap on Shihan (model teacher). Something like model teacher's model. In Japan most Aikikai groups use the term Kaiso (founder) which makes more sense and makes Ted's point irrelevent as it does not imply superior anything.