The low-rated anomalies at the beginning are the Clip Show eps, hardly a surprise there.
That really low rated one towards the end of the graph is the Gaga episode, again not a surprise in the least.
According to IMDB, the two greatest episodes of all time are ironically the two episodes that immediately precede the one that broke the fanbase for good (Principal and the Pauper).
The highest rated episodes after S9 are:
>Trilogy of Error (S12E18) - 8.6/10
>Eternal Moonshine of the Simpson Mind (S19E09) - 8.2/10
>Holidays of Future Passed (S23E09) - 8.2/10
>Barthood (S27E09) - 8.3/10
And those would only place in the middle of the pack when you look at how well the first eight seasons ranked, which goes to show how the quality dropped over time.