Zerdini, who sometimes comments here, recently pointed me to an article he wrote about a 1933 séance involving direct-voice medium William Cartheuser.
I haven't researched this event on my own -- actually I'd never heard of Cartheuser until now -- but the story certainly makes interesting reading and, on the face of it, sounds like remarkably good evidence.
However, in Googling "William Cartheuser," I found that some people who accepted other mediums as genuine nevertheless apparently regarded Cartheuser as a fraud. One was Hereward Carrington, and another was Nandor Fodor.
A balanced view of the subject is provided in this excerpt from The Case for Life after Death, by Elizabeth E. McAdams and Raymond Bayless, which is online here:
Bayless sat a number of times with William Cartheuser, a physical medium who gained considerable notoriety in his day. He apparently produced phenomena by trickery as often as he could. He used a flashlight equipped with a pinhole for "spirit lights"; he squirted water about with a water pistol; he picked up his trumpets and whirled them about with masterly skill; he calmly reached about, creating "spirit touches"; and he even walked about the séance room producing phenomena. But in spite of these habits he possessed genuine telepathic ability and in all probability had some authentic ability as a physical medium as well. Additional discussion of Cartheuser can be found in Hereward Carrington's The Invisible World. [p. 54]
Luckily, Carrington's book is online, and although it is only a "limited preview," the entire section on Cartheuser -- pages 68-93 -- is available. Carrington clearly had very good grounds to be suspicious of Cartheuser, yet he did conclude that "a small residuum" of the phenomena were worthy of further study. This residuum included uncannily accurate touches (by the trumpet) in a pitch-dark room; a few instances of overlapping voices produced by the medium; isolated bits of information from Cartheuser that seemed impossible for him to have known by any normal means; and one test in which a light was rigged to go off if Cartheuser left his chair. In this test, the light stayed on throughout the séance, yet "touches and other phenomena occurred nevertheless at considerable distances" from the chair.
These indications of genuine mediumship are balanced by very obvious signs of deception on many occasions, as recounted in Carrington's book.
At the end of the day, I don't have enough information to evaluate Cartheuser. My impression is that he might be similar to Eusapia Palladino (a comparison also made in the McAdams-Bayless book): someone with genuine abilities but a tricksterish personality, who freely used deception when possible.
In any event, I enjoyed reading about the "séance of the century" and thought others might, also.