Graham Hancock -- Some of his theories aren't exactly rock solid (like the Orion Correlation) but some are plausible, and he's always got interesting things to say about Zahi Hawass and the reigning authorities at Egypt's archeological sites.
Graham Phillips -- The second of the British Grahams, this guy does his research. I loved the show where he talked about how the volcanic explosion on Thera (Santorini) could have been a real-life explanation for the phenomena described in the Plagues of Egypt.
Michio Kaku -- You gotta love a physicist who will spend five hours fielding calls about the most insane things imaginable, and who can have a long conversation about how the Q from Stark Trek would be "a type III civilization." I especially love his discussions about time dilation and humanity's possible future as a spacefaring civilization.
Terence McKenna -- It's tempting to lump the late McKenna into the same category as the Richard Hoaglands and Jose Escamillas, because he talks about some insane things. But I always found him to be humble and always willing to entertain differing opinions, a trait you rarely see in people as brilliant as he was. Terence was a nice guy, a gentle soul, and a great guest for Coast to Coast.