Having started in Ki Society and trained around quite a bit in most many other schools of aikido I don't see any style having a monopoly on doing O'Sensei's aikido and those that tend do be doing the best seem to have broken from the mould/constraints of their style somewhat.
Note that not everyone is claiming to do the founder's Aikido, Tohei in particular was quite clear on this point:
"The one essential thing I learned from Ueshiba Sensei was how to relax. He was always relaxed in the face of conflict, which is why his Aikido was so strong. He would do this himself, but he encouraged his young students to hold with as much strength as possible. In Aikido if you are not relaxed you cannot throw a person. It seemed a mystery to us that Ueshiba Sensei could always throw, could always get out of a hold. He would lead your Ki, and could always throw his opponent in the direction he was already going. I began to make rapid progress after I started copying what he did, and paid less attention to what he said. I ended up only keeping about 30% of the techniques I learned from Ueshiba Sensei, changing or dropping the rest. What I really learned from him was not technique, but the true secret of Aikido, non-dissension; not to resist your opponent's strength but to use it."
the complete interview is here: