Beat me to it. :) That's probably why Rush uses him. He makes you want Rush back.
I tolerate Hedgecock when he's subbing for Rush, but my favorite guest host was always Tony Snow. Liberal kooks always jammed the phones when he was on. Hannity was also good, before he became the second highest rated show behind Maharushie and started hawking his books and kissing up to pols on the air.
A few people have mentioned Chrissy Matthews one time as guest host, and that was definitely the worst. Another one timer who hasn't been mentioned was Tim Russert, and he was actually pretty good.
Two people who have never filled in, but should, are Laura Ingraham and Ann Coulter. Talk about slice and dice a RAT!