Before you could even defend yourself against an attacker, you should know yourself first.
I'm not the easiest uke to perform a technique on, and I'm not that much of a nage. I attack seriously with maintaining control, keeping one point, one mind and body, and try to extend the Ki. Since I do not attack blindly, that makes me quite a hard uke to practice a technique on.
At the same time, people that also understands this and becomes my uke, they too become a strong uke. It is not easy to perform techniques when they maintain this composure. This is the time when Aiki is really needed, because strength becomes nothing here, no matter how strong you are.
Would I be able to throw myself? Eventually, with proper aiki, yes. Because as my own uke, I know my own strength and weaknesses and I could use that to stabilize myself all the way through a technique. This would make it the hardest uke ever.
I basically have a twin (not in a literal manner but in a training kind of sense) in the dojo also.