After tabulating my hunches, it looks like the 1997-1998 season (#9) is a prime candidate for the first season where the bad significantly outweighed the good. The first three episodes are the Homer in New York, the two Principal Skinners, the screedy “Homer gets a gun” episode, Homer coaching the pee-wee football team (featuring a crossover with the new King of the Hill show)… all terrible. However, this isn’t when Mike Scully took over for Conan O’Brien, a popular theory among people even geekier than I… according to this interview, he started in Season #5 (1993-1994).
This will, no doubt, hearten those of us who feel exactly as Norbizness does, especially in relation to the deplorable 4F19. Read comments for other suggestions, none of which I can really disagree with (except anyone who says that any episode after season 8 was any good; they’re lying sacks of crap smoking high-quality dope).
I probably take the Simpsons too seriously. Probably.