That can be a very difficult answer as some other aikido schools usually dont like the answer that is given. Some people would say that Iwama is the closest to the founders tech/ Or Iwama is the least changed form of Aikido. Also some would say that Iwama is what the founder was doing, While the hombu had more freedom to change through doshu and tohei. Iwama is also the home of the basics and the weapons system of aikiken/jo was born there, and handed down to saito sensei to preserve and develop. You need to spend some time looking on the net for iwama sites.
here are some with good info
also check other sites in search engines