1998 - Charlie Rose - Interview with Larry David
He says that the episode called "The Contest" (in the 4th season) was one of the major things that put them over the top; it got tremendous word-of-mouth.
https://www.youtube.com/watch?v=LyGcnGn7mRk
In this interview with Jason Alexander, Jason says that the watercooler episode was the one that really put them on the map.
2015.01.13 - NYT - TimesTalks - Larry David
He gives a good description of how he became successful.