Do you mean seen personally, i.e. being there in the dojo watching/ experiencing, or do you mean seen on video/ DVD, or combination of both?
For me being there personally in class, in no particular order, Nakao Sensei, Endo Sensei, Ueshiba Moriteru Doshu, Tada Sensei, Matsuda Sensei, Nojima Sensei, Koyama Sensei, Uchikoshi Sensei, Osawa Sensei, Tanaka Sensei. I've been uke to all except Tada Sensei, Toyoda Sensei and Matsuda Sensei. I'm not trying to write a who's who list, or look at who I've trained with, but they have all been excellent at what they do and thoroughly enjoyable to train with and learn from. There are many others too, especially here in the UK, Bob Spence, Alan Sanders, Aubrey Smith, Tom Moss, Keith Downs.
On video, of course Ueshiba Morihei and Ueshiba Kisshomaru, Saotome Sensei, Yamada Sensei, Chiba Sensei, Yamaguchi Sensei, Tissier Sensei, Isoyama Sensei, Shioda Gozo Sensei, Fujita Sensei and Nishio Sensei to name a few, and more that I've forgotten.
There are so many good exponents of the art above that I couldn't even begin to compare them, list them in some kind of order of bestness, or choose one. But, if I had to say which one inspires me most, ruling out O Sensei, it would be Yamaguchi Sensei and his students, Endo Sensei and Nakao Sensei
Bryan (the namedropper)