How about Andrew Collins? No, I don't mean Colin Andrews, I'm referring to the prolific UK-based author who has written books on many aspects of the paranormal. Some of his theories are extremely controversial - for example, he tends to think that UFOs are organic creatures rather than spacecraft, which makes him one of Trevor James Constable's few remaining adherents. He has also done a fair bit of research in Egypt, which brought him into initial uneasy cooperation followed by acrimonious conflict with that delightful man, Dr. Zahi Hawass. I'm sure Andrew has a few stories to tell about that!
He also has an elaborate theory that human evolution was kick-started by high-energy particles from the constellation Cygnus, which stone age man somehow detected, resulting in a worldwide cult of swan worship. Also, on 21.12.12 the Sun will go out, but it will almost immediately be replaced by another one which will pop out of a "hyperspace birthing tube", though that idea was almost entirely excised from his published book on the subject, presumably because his editors considered it to be a bit much.
Also, Andy has a long history of involvement in the Psychic Questing movement. I don't know if this happens in the USA, but basically, the idea is that psychic people (Andy himself is in no way psychically gifted and cheerfully admits it, but many of his friends are) search for amazing magic artifacts which they actually find, thus preventing terrible disasters such as the birth of the Antichrist. It should also be noted that Andy and his mates, at an early stage in their careers, foiled a sinister fellow known as the Black Alchemist, who became sufficiently well-known to enter the realm of fiction, and subsequently had his ass kicked by both Lara Croft and Green Arrow.
I'm suggesting Andy because he's had an incredibly varied career as an all-round paranormalist, some of which is frankly a little bit hard to swallow, but the man himself comes across as being surprisingly sincere. You've had a few guests who were blatantly making it all up, and some who didn't seem to know very much about their own subject (take a bow, Mr. Sanchez), but Andrew Collins, though perhaps a little too quick to jump to amazing conclusions, is the real deal. From reading his work, I get the impression that he is perfectly sane, and lacks the pomposity that mars far too many books of this type. Probably his worst failing is an overdeveloped sense of wonder at the expense of critical thinking, but around here, is that even a failing?
I think that if you booked Andy, you'd have a guest with a very wide range of interests who really has done a great deal of research, and who would be fully prepared to go out on a limb and talk about wildly unlikely things because he's genuinely sincere. Also, judging by his books, he seems to be quite a nice chap. Though he may get a bit stroppy if you mention the Brentford Griffin...