3. Steve Martin
On Martin's final appearance on Late Show last week, Letterman told the writer-actor, "I think you and I are strange in the same way." It's easy to see why: Both men love absurdist humor, abhor Hollywood phoniness and wield a withering wit. (Also, Johnny Carson adored them both, having Martin guest-host The Tonight Show.) Watching Martin's appearances on Late Night and Late Show over the years was to see the comic evolve from budding movie star to Renaissance man, from jokester to refined satirist. Whether devoting a whole segment in 1987 to teaching Dave how to avoid card cheats or producing the classic 1998 clip "Dave and Steve's Gay Vacation," Martin always took his appearances seriously, which guaranteed they'd be hilarious.