Comedians?
If a comedian is going to make it to the "top rung," he/she has to be more than just a comic. He/she has to be an all-round entertainer and a pretty good actor, because a comic's schtick gets old after awhile. Perfect example: my late former publicity client, Stanley Myron Handleman, who was very hot during the late 1960s/early 70s, but a few years later, couldn't get arrested. He had one act, and funny as it was, audiences got tired of it.
BTW: I love Ricky Gervais. I watched his HBO Special the other night, and I couldn't stop laughing.