18. "St. Elsewhere" (NBC) — 22.5 million viewers

"St. Elsewhere" was a groundbreaking urban medical drama that aired on NBC from 1982 to 1988. Its alumni include Denzel Washington, Ed Begley Jr., Howie Mandel, and Mark Harmon. Viewers tuned into its May 25, 1988, series finale to find out that the whole series took place in the very active imagination of a young autistic boy!