My personal favorite fill in host though is without a doubt Walter Williams. I love the man's columns and his thought process' (e.g. "how do you cook a frog"). He's a treasure.
Out of the few stand-ins I've listened to, I liked Roger Hedgecock the most.
Walter E Williams is also my favorite, but because he doesn't get used as much as the others.
The sad part is mark belling is usually much better doing the local show than he is filling in for rush. As a sub he's more restrained and careful. With his own show he'll go off on tangents and get a little weird, but that's what makes him interesting to listen to.
What is Walter Williams like?
I have this image of his wife standing in the doorway, tapping her foot with rolling pin in hand, waiting for him to get home after subbing for Rush.