By popular demand, here are a few more examples of Jack Benny's greatness.
A number of commenters mentioned the famous "Sy" routine with Mel Blanc. Notice the timing... from both Mel and Jack.
Other commenters noted his influence on Johnny Carson. Here they are together.
And finally, Jack and his violin. This is a segment of a tribute to Benny hosted by another one of his disciples, Kelsey Grammer.