4. Howard Stern
No wonder David Letterman and Howard Stern saw each other as kindred spirits. Both proudly idiosyncratic, both refusing to kowtow to corporate bosses, they were two peas in a grouchy pod. What made Stern's appearances great in the Late Night days was that his button-pushing provocations helped give Dave's show its edge. (In 1984, Stern wondered how Vice President George H.W. Bush could get aroused being married to the much-older-looking Barbara Bush.) Few stars stood up for Letterman as defiantly as Stern did during the late-night wars, and even though the two men had a falling out later, they renewed their friendship before Dave's retirement. Those final Late Show appearances allowed viewers to enjoy some in-depth interviews and be reminded how good both men were in drawing out their guests through interesting, engaged conversation.