100% agreement. I enjoy him nearly as much as I enjoy Rush, though I'm sure I wouldn't enjoy listening to him day in and day out as I do Rush. The thing about the other fill-in hosts is that they're just lesser versions of Rush. But Walter Williams is his own thing. He has a technical expertise that Rush doesn't have, so he kind of fills a void. He's like an essential nutrient that you need every so often that Rush doesn't supply.
After that, I'd say Roger Hedgecock is the best, but he's a step down, and it's a pretty steep rolloff from there.
Bottom line is that Rush is irreplaceable. I don't what we're going to do when he's gone.
The plan is to eventually have him become more and more cybernetic as hit parts fail so he never dies and can keep doing his job. That is also the plan with Dick Cheney, BTW. Both of these men are cyborg and will become increasingly so.