9. Robin Williams
The knock on Williams was that his inspired, high-energy talk-show segments started to become a crutch, merely an endless string of non-sequitur impersonations and undisciplined manic riffing. But especially since his suicide in 2014, it's impossible not to view those appearances with far more sympathy, recognizing the self-loathing and sadness that fueled his nonstop motion. When Letterman returned to Late Show after heart surgery in 2000, Williams was his first guest, coming out dressed in scrubs and surgical mask. ("Aren't you glad you didn't see this at the surgery?" Williams joked to the host. "'Hey, Dave! It's me!!'") Dave and Robin had known each other for 38 years when Williams died, first meeting at the Comedy Store in Los Angeles. "It's like nothing we had ever seen before, nothing we had ever imagined before," Letterman would say after Williams' passing about those early days of watching Robin's stand-up sets. "We didn't approach him because we were afraid of him, honest to god. You thought, 'Holy crap, there goes my chance at show business.'"