When Rhoda met Sally
I just watched the episode from Season 3, when they introduced the character of Sally Gallagher. This may be when the show officially 'jumped the shark'.
The character was horrible, not likable or believable at all. To try and pass Anne Meara (well into her 40s by that time) as a sexy 39 year old stewardess that men hit on all the time was ridiculous. The whole set-up of how they met (roomies at a single's weekend) was even more far-fetched (though you could see they were trying to go for the same set-up as when Rhoda and Mary met - both thinking the apartment was 'theirs'...same thing with the hotel room). The whole situation fell flat, as does the horrible character they came up with.
"I prefer fantasy over reality TV - like Fox News" - B.Streisand