I enjoy quite a few of the guests already mentioned, especially Jim Marrs (as others have said, I don't believe his theories, necessarily, but enjoy the heck out of hearing him tell the stories); Michio Kaku; the alien implant doctor; GIS; etc.
And even though it was a George interview, I liked whoever it was that discussed the young boy allegedly reincarnated from a World World II pilot, the one who kept saying something like, "Little man on fire!" and giving details about a WWII plane he should have had no way of knowing about.
I also remember really liking a writer Ian has had on a few times, Mary Roach, who wrote "Packing for Mars." She's pretty funny and down to earth, and she and Ian have a good rapport, so her interviews are anything but dry presentations of science.
Another one (I think of Ian's, because I enjoyed the interview, so hard to believe it was Snoory) is astrophysicist Jeffrey Bennett, who wrote the series of kids books featuring a Rottweiler who visits various planets.