The 2001 World Series telecasts (New York Yankees vs. Arizona Diamondbacks) claimed the largest U.S. TV audiences in recent memory: 24.5 million viewers per telecast, on average. 
But the late Seventies and early Eighties remain the true glory days of World Series TV viewership.
The 1980 World Series telecasts — the last time the Philadelphia Phillies won a World Series — drew the largest per game TV audiences on record: almost 54.9 million average viewers.