I think it is hard to judge how good Ueshiba was since he gained much of his reputation on his pre WWII aikido where I am certain he was quite a lethal martial artist and extremely strong. I'd question whether Shioda was better than Ueshiba, since I am under the impression that Shioda himself didn't believe that, and he was a very candid person.
When we say who is best at aikido, it depends on what you think aikido is. I believe Ueshiba was one of the best martial artists who lived (though I think he was not against bolstering his own image). I also believe that aikido is not the whole of what Ueshiba was about - he was insanely dedicated and very physcially powerful. Also, I don't think he transferred his complete knowledge to anyone.