TV show whose finale holds the record for the most viewed episode in TV history:

MASH
The final episode of MASH, which aired in 1983, was viewed by 50.15 million households.
Source: Atlanta Journal-Constitution, 2/12/2006

 
   
  The second most watched T.V. episode of all time:

Dallas (Who shot JR)
The "Who Shot J.R.?" episode of Dallas in 1980 was watched by 53% of households with tv's, or 83.6 million people.
Source: USA Today, 5/18/1998

 
   
  Most watched TV episode of all time:

MASH final episode
The final episode was watched by 60.2% of American homes with TV sets, or 106 million viewers, nearly half the U.S. population at the time.
Source: Entertainment Weekly, 5/1/1998

 
   
  The third most watched T.V. episode of all time:

Roots
ABC's "Roots," Part 8, aired on Jan. 30, 1977, and was watched by 51.1% of people with televisions.
Source: Christian Science Monitor, 5/8/1998