I nominate Mark Steyn - the best Rush substitute on the air, bar none. I know that there are others that are good, but very few get my ear the way Rush did, and Steyn does. He's brilliant in his own way.
Without a doubt he is one of the first names to come to mind. Besides confirmation that he would commit to 15 hours 'live' every week, I find occasionally Steyn can get overly involved with a leftist caller past the point of being interesting. Still, Steyn is good listening, generally.
I would like to have Steyn be the host too. He was my favorite guest host.