The main reason Aikidoka don't admit their prowess is:
1. Words are worthless.
2. If we should say such a thing, than we may start to believe it.
3. Anyone who says such a thing is usually pretty clueless about budo.
I'm a lot better than I was a year ago, a month ago, even perhaps an hour ago, but to say I was the best budoka around would only make my training slack and make the challenges come rolling in the door, which is (IMO), the main thing to avoid in Aikido...